The distance between successive points of maximum pressure (or, which is the same thing, between successive points of minimum pressure) is the wavelength. The number of waves emitted in one second is the frequency. To illustrate: a tuning fork that sounds a tone corresponding to middle C on the piano is vibrating 264 times a second. Each second, 264 areas of high pressure followed by areas of low pressure are produced. The frequency is therefore 264 cycles per second. During that second, sound has traveled 1090 feet (if the temperature is at the freezing point). If 264 high-pressure areas fit into that distance, then the distance between neighboring high-pressure areas is 1090 divided by 264, or about 4.13 feet. That is the wavelength of the sound wave that gives rise to the sound we recognize as middle C.
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THE EXTERNAL AND MIDDLE EAR
We can see that there is nothing mysterious about the conversion of soVtnd waves to nerve impulses. From one point of view, hearing is a development and refinement of the pressure sense. Sound waves exert a periodic pressure on anything they come in contact with. The pressure is extremely gentle under ordinary circumstances, and a single high-pressure area associated with sound would not affect the ear, let alone any other part of the body. It is the periodic nature of the sound wave, the constant tapping, so to speak — not the tiny pressure itself, but the unwearying reiteration of the pressure according to a fixed pattern — that sparks the nerve impulse. A fish hears by means of sensory cells equipped to detect such a pressure pattern. These sound-receptor cells are located in a line running along the mid-region of either side and are referred to as the lateral lines.
The emergence of vertebrates onto land created new problems in connection with hearing. Air is a much more rarefied medium than water is, and the rapid periodic changes in pressure that represent the sound waves in air contain far less energy than the corresponding pressure changes of sound waves in water do. For this reason, land vertebrates had to develop sound-receptors more delicate than the fish's lateral line.
The organ that underwent the necessary development is located in a cavity of the skull on either side of the head, this small cavity being the vestibule. In the primitive vertebrates, the vestibule contained a pair of liquid-filled sacs connected by a narrow duct. One is the saccule (sak'yool; "little sack" L), the other the utricle (yoo'trih-kul; "little bottle" L}. For all vertebrates, from the fish upward, these little organs in the vestibule represented a sense organ governing orientation in space, something which may be referred to as the vestibular sense. The utricle and its outgrowths remain concerned with the vestibular sense in all higher vertebrates, too, including man, and I shall describe this later in the chapter.
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From the saccule, however, a specialized outgrowth developed in land vertebrates. This was adapted as a sound-receptor, one that was much more sensitive for the purpose than the lateral-line cells of the fish. It remained, then, to transmit the sound waves from air to the new sensing organ within the vestibule. For this purpose, the parsimony of nature made use of the hard structure of some of the gills, which, after all, were no longer needed as gills in the newly emerging land vertebrates. Tbe first gill bar, for instance, was altered into a thin diaphragm that could be set to vibrating very easily, even by pressure alterations as weak as those of sound waves in air. Another gill bar became a small bone between the diaphragm and the sound-receptor, and acted as a sound transmitter.
With the development of mammals, a further refinement was added. The mammalian jaw is much simpler in structure (and more efficiently designed) than is the ancestral reptilian jaw. The mammalian jaw is constructed of a single bone rather than a number of them. The reptilian jawbones that were no longer needed did not entirely disappear. Some were added to the nearby sound-sensing mechanism (check for yourself and see that your jawbone extends backward to the very neighborhood of the ear). As a result, there are three bones connecting diaphragm and sense-receptors in mammals instead of the one of the other land vertebrates such as the birds and reptiles. The three-bone arrangement allows for a greater concentration and magnification of sound-wave energies than the one-bone does.
Let me emphasize at this point that the hearing organ is not what we commonly think of as the ear. What we call the ear is only the external, clearly visible, and least important part of the complex system of structures enabling us to hear. Anatomically, the visible ear is the auricle ("little ear" L), or pinna" (see illustration, p. 253).
* The word "pinna" is from a Latin expression for "feather" and can be used for any projection from the body. "Fin," for eiample, is derived from the same root as pinna. The application to the ear is fanciful, but reasonable.
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The auricle is another uniquely mammalian feature. In many mammals it is trumpet-shaped and acts precisely as an old-fashioned earphone does; an effect carried to an extreme in such animals as donkeys, hares, and bats. The ear trumpet collects the wave-front of a sound wave over a comparatively broad area and conducts it inward toward the sound-receptors, the sound wave intensifying as the passage narrows (much as the tide grows higher when it pours into a narrowing bay like the Bay of Fundy). By use of such a trumpet, which, moreover, is movable so that sounds can be picked up from specific directions, the hearing organ of mammals is made still more sensitive. By all odds, then, the mammals have the keenest sense of hearing in the realm of life.
In man, and in primates generally, there is a recession from the extreme of sensitivity. The trumpet shape is lost, and the auricle is but a wrinkled appendage on either side of the head. The outermost edge of the auricle, curving in a rough semicircle and folding inward, sometimes yet bears the traces of a point that seems to hark back to an ancestry in which the ear was trumpet-shaped. Charles Darwin used this as one of the examples of a vestigial remnant in man that marked lower-animal ancestry. The ability to move the ear is also lost by primates, but in man three muscles still attach each auricle to the skull. Although these are clearly intended for moving the ear, in most human beings they are inactive. A few people can manage to work those muscles slightly, and by doing so can "wiggle their ears," another clear harking back to lower-animal ancestry.
This shriveling of the ear trumpet in man is usually considered an indication of the growing predominance of the sense of sight. Whereas some vertebrates grew to depend upon their ears to detect the slightest sound, perhaps to warn of an enemy, the developing order of primates threw more and more weight on their unusually efficient eyes. With alert eyes flashing this way and that, it became unnecessary to waste effort, one might say, in moving the ear, or in bearing the inconvenience of long
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auricles for the purpose of unnecessarily magnifying extremely faint sounds. Nevertheless, though we do miss a faint sound that might cause a dog to prick up its ears, our shriveled auricle does not really indicate any essential loss of hearing. Within the skull, our hearing apparatus can be matched with that of any other mammal on an all-round basis.
In the center of the human auricle is the opening of a tube which is about an inch deep, a quarter of inch in diameter, reasonably straight, and more or less circular in cross section. This is the auditory canal, or the auditory meatus (mee-ay'tus, "passage" L). Together the auricle and the auditory canal make up the external ear. Sound collected by the auricle is conducted through the auditory canal toward the vestibule. The canal is lined with some of the hardest portions of the cranium, and the functioning parts of the ear — the actual sound-receptors — are thus kept away from the surface and are well protected indeed. Birds and reptiles, which lack auricles, do have short auditory canals and so do not lack external ears altogether.
The inner end of the auditory canal is blocked off by a fibrous membrane, somewhat oval in shape and 1/10 of a millimeter (about 1/250 inch) thick. This is the tympanum (tim'puh-num; "drum" L), or tympanic membrane.' The tympanum is fixed only at the rim, and the flexible central portion is pushed inward when the air pressure in the auditory canal rises and outward when it falls. Since sound waves consist of a pattern of alternate rises and falls in pressure, the tympanum moves inward and outward in time to that pattern. The result is that the sound-wave pattern (whether produced by a tuning fork, a violin, human vocal cords, or a truck passing over a loose manhole cover) is exactly reproduced in the tympanum, As the name implies, the tympanum vibrates just as the membrane stretched across a drum would, and its common name is, in fact, the eardrum.
Along the edges of the tympanum are glands which secrete a
* This is the diaphragm I described on page 249 as having been developed out of the first gill bar of our fishy ancestors.
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soft, waxy material called cerumen (see-roo'men; "wax" L), though earwax is its common name. This serves to preserve the flexibility of the tympanum and may also act as a protective device. Its odor and taste may repel small insects, which might otherwise find their way into the canal. The secretion of cerumen is increased in response to irritation and the wax may accumulate to the point where it will cover the tympanum and bring about considerable loss of hearing until such time as the ear is washed out.
On the other side of the tympanum is a small air-filled space called the tympanic cavity. Within that cavity are three small bones to conduct the vibrations of the tympanum still farther inward toward the vestibule. Collectively the three bones are the ossicles ("little bones" L). The outermost of the three ossicles is attached to the tympanum and moves with it. Because in doing so it strikes again and again on the second bone, this first ossicle is called the matteus (mal'ee-us; "hammer" L). The second ossicle which receives the hammer blows is the incus ("anvil" L).
The incus moves with the malleus and passes on the vibrations to the third bone, which is shaped like a tiny stirrup (with an opening not much larger than the eye of a needle) and therefore called the stapes (sta/peez; "stirrup" L).* The inner end of the stapes just fits over a small opening, the oval window, which leads into the next section of the ear. The whole structure from the tympanum to this small opening, including the tympanic cavity and the ossicles, is called the middle ear.
The function of the ossicles is more than that of transmitting the vibration pattern of the tympanum. The ossicles also control intensity of vibration. They magnify gentle sounds, because the oval window is only 1/20 the area of the tympanum and sound waves are once more narrowed down and in that way
" The malleus and the incus are the remnants of bones that in our reptilian ancestors were to be found in the jaw, as I explained earlier. They are found in the mammalian ear only. The stapes originated from one of die gill bars of the ancestral fishes, and is found in the ears of birds and reptiles as well as in those of mammals.
SEMICIRCULAR CANALS /
SIDE VIEW OF EARDRUM
FRONT VIEW OF EARDRUM
MALLEUS
OSSICLES OF THE EAR
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intensified. In addition, the lever action of the ossicles is such that sound-wave energy is concentrated. The net result is that in passing from the tympanum to the oval window sound is amplified as much as fiftyfold.
The ossicles also damp out loud sounds. Tiny muscles that extend from the malleus to the skull place tension on the tympanum and prevent it from vibrating too strenuously; an even tinier muscle attached to the incus keeps the stapes from pressing too vigorously against the oval window. This action of magnification-and-damping extends the range of loudness we can hear. The loudest sounds we can hear without damage to the ears result from sound waves containing about 100 trillion times the energy of the softest sounds we can just barely make out. These softest sounds, by the way, result from movements of the eardrum of a two-billionth of an inch, and this represents far less energy than that in the faintest glimmer of light we can see. From the standpoint of energy conversion, then, the ear is far more sensitive than the eye.
Sound waves are conducted through the bones of the skull, but the ossicles do not respond to these with nearly the sensitivity with which they respond to tympanic movements, and this is also most helpful. Were they sensitive to bone vibration we would have to live with the constant rushing sound of blood through the blood vessels near the ear. As it is, we can hear the blood-noise as a constant hum if there is a reasonable silence and we listen carefully. This sound can be magnified by a cupped hand, or, traditionally, by a seashell; children are told that it is the distant roaring of the sea.
This filtering out of bone-conducted sound also means that we are not deafened by the sound of our voice, for we do not hear it through bone conduction chiefly, but through the sound waves carried by the air from mouth to ear. Nevertheless, bone conduction is a minor factor and adds a resonance and body to our voice we do not hear in others. When we hear a recording of our own voice, we are almost invariably appalled at its seem-
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ing inferior quality. Even the assurance of bystanders that the recording is a precise reproduction of our voice leaves us somehow incredulous.
The ossicles can sometimes be imperfect in their functioning. If the tiny muscles attached to them are damaged, or if the nerves leading to those muscles are, the ossicle movements become somewhat erratic. There may be needless vibration (something like the frame of an automobile plagued with a loose bolt). In that case, there is a continuous sound in the ears (tinnitis- "jingle" L), which can be endlessly irritating.
From the middle ear a narrow tube leads to the throat. This tube is called the Eustachian tube" (yoo-stay'kee-an) after the Italian anatomist Bartolommeo Eustachio, who described the structure in 1563; see illustration, page 253. The middle ear is thus not truly within the body, but is connected to the outer world by way of the throat. This is important, because the tympanum will move most sensitively if the air pressure is the same on both sides. If the air pressure were even slightly higher on one side than on the other, the tympanum would belly inward or outward. In either case it would be under a certain tension and would then move with lesser amplitude in response to the small pressure changes set up by sound waves.
The pressure of our atmosphere changes constantly through a range of 5 per cent or so, and if the middle ear were closed, the pressure within it would rarely match the changing air pressure in the auditory canal. As it is, however, air flows in and out through the Eustachian tube, keeping the pressure within the tympanic cavity continually equal to that in the auditory canal. When external air pressure changes too rapidly, the narrow bore of the Eustachian tube is insufficient to keep the inner pressure in step. The pressure difference then causes a pressure on the tympanum which is uncomfortable and can even be painful. Everyone who has traveled up or down in a rapid elevator knows the sensation. Swallowing or yawning forces air through the
• The Eustachian tube evolved from the first gill slit of the ancestral fish.
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Eustachian tube in one direction or the other and relieves the condition.
When the Eustachian tube is closed through inflammation during a cold, the resulting discomfort is less easily relieved, and is one more addition to the annoying symptoms of this most common of infectious diseases. The Eustachian tube offers a route, too, whereby bacteria can penetrate the recesses of the skull and find there a perfect haven. Such middle ear infections are more common in children than in adults. They are painful and hard to treat (though the coming of antibiotics has helped}, and can be dangerous.
THE INTERNAL EAR
On the other side of the oval window covered by the stapes is the vestibule referred to on page 248. Both the vestibule and the structures within it are filled with a thin fluid much like the cerebrospinal fluid. Here the sound waves are finally converted from vibrations in air to vibrations in liquid. It is to the latter that the hearing sense was originally adapted in the first vertebrates, and the whole elaborate structure of the outer and middle ear is designed, in a way, to convert air vibrations to liquid vibrations with maximum efficiency.
There are two organs in the vestibule. Lying above and forward are the utricle and the structures developed from it, and lying below and behind are the saccule and the structures developed from it. All the contents of the vestibule are lumped together as the internal ear, but only the saccule portion is concerned with hearing. The utricle and attendant structures are concerned with the vestibular sense, and I leave them to one side for now.
The tube that in land vertebrates developed from the saccule is the cochlea (kok'lee-uh; "snail-shell" L), which is a spiral structure that does indeed have a close resemblance to a snail shell, except that its width does not narrow as it approaches its central apex, but remains constant (see illustration, p. 253).
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The acoustic nerve leads from the cochlea. It is the cochlea that contains the sense-receptors making it possible for us to hear. The cochlea is not a single coiled tube but, rather, is a triple one, all coiling in unison. The upper part of the cochlea, which leads from the stapes and the oval window, consists of two tubes, the vestibular canal and the cochlear canal, separated by a very thin membrane. This membrane is too thin to block sound waves, and so, for hearing purposes, the two tubes may be considered one. The lower half of the cochlea is the tympanic canal. Between this and the double tube above is a thick basilar membrane (bas'ih-ler; "at the base" L). The basilar membrane is not easily traversed by sound waves.
Resting on the basilar membrane is a line of cells which contain the sound-receptors. This line of cells was described in 1851 by the Italian histologist Marchese Alfonso Corti, and so it is often called the organ of Corti. Among the cells of the organ of Corti are hair cells which are the actual sound-receptors. The hair cells are so named because they possess numerous hairlike processes extending upward. The human organ of Corti is far richer in such hairs than is that of any other species of animal examined. Each cochlea has some 15,000 hairs altogether. This seems reasonable in view of the complexity of the speech sounds human beings must listen to and distinguish. Delicate nerve fibers are located at the base of the hair cells. These respond to the stimulation of the hair cells by the sound waves and carry their impulses to the auditory nerve, which in turn transmits its message, via various portions of the brain stem, to the auditory-center in the temporal lobe of the cerebrum.
An interesting question, though, is how the cochlea enables us to distinguish differences in pitch. A sound wave with a relatively long wavelength, and therefore a low frequency, is heard by us as a deep sound. One with a relatively short wavelength and high frequency is heard by us as a shrill sound. As we go up the keyboard of a piano from left to right we are producing sounds of progressively shorter wavelength and higher
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frequency, and though the progression is in small steps we have no difficulty in distinguishing between the tones. We could even distinguish tones that were more closely spaced — represented on the keyboard by the cracks between the piano keys, one might say.
To solve the problem of pitch perception, the cochlea must be considered in detail. The sound waves entering the cochlea by way of the oval window travel through the fluid above the basilar membrane. At some point they cross the basilar membrane into the fluid below and travel back to a point just beneath the oval window. Here there is an elastic membrane called, from its shape, the round window. Its presence is necessary, for liquid cannot be compressed, as air can. If the fluid were in a container without "give," then sound waves would be damped out because the water molecules would have no room to push this way or that. As it is, though, when the sound waves cause the stapes to push into the cochlea, the round window bulges outward, making room for the fluid to be pushed. When the stapes pulls outward, the round window bulges inward.
One theory of pitch perception suggests that the crux lies in the point at which the sound waves are transmitted from the upper portion of the fluid across the basilar membrane into the lower portion. The basilar membrane is made up of some 24,000 parallel fibers stretching across its width. These grow wider as one progresses away from the stapes and the oval window. In the immediate neighborhood of the oval window the fibers are about o.i millimeters wide, but by the time the far end of the cochlea is reached they are some 0.4 millimeters wide. A fiber has its own natural frequency of vibration. Any frequency may, of course, be imposed upon it by force, but if it is allowed freedom, it will respond much more vigorously to a period of vibration equal to its natural period than to any other. This selective response to its natural period of vibration is called resonance. Of two objects of similar shape, the larger will have a lower natural frequency. Consequently, as one travels along the basilar
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membrane its resonance will respond, little by little, to lower and lower frequencies.
It was tempting to think that each type of sound wave crossed the basilar membrane at the point where the resonance frequency corresponded to its own. High-pitched sounds with short wavelengths and high frequencies crossed it near the oval window. Deeper sounds crossed it at a greater distance from the oval window; still deeper sounds crossed it at a still greater distance, and so on. The hair cells at the point of crossing would be Stimulated and the brain could then interpret pitch in accordance with which fibers carried the message.
This theory would seem almost too beautifully simple to give up, but evidently it has to be abandoned. The Hungarian physicist Georg von Bekesy has conducted careful experiments with an artificial system designed to possess all the essentials of the cochlea and has found that sound waves passing through the fluid in the cochlea set up wavelike displacements in the basilar membrane itself.
The position of maximum displacement of the basilar membrane — the peak of the wave — depends on the frequency of the sound wave. The lower the frequency, the more distant the peak displacement is from the oval window, and it is at the point at which this peak is located that the hair cells are stimulated. The alteration of the form of displacement of the basilar membrane with pitch does not seem to be great. However, the nerve network can, apparently, respond to the peak of the wave without regard to lesser stimulations near by and can record slight changes in the position of that peak with remarkable fidelity. (In a way, this is similar to our ability to "listen," that is, to hear one sound to which we are paying attention, while damping out the surrounding background noise. We can carry on a conversation in a crowd in which many are talking simultaneously, or amid the roar of city traffic.)
Naturally, any given sound is going to be made up of a variety of sound waves of different frequencies, and the form of the
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26l
displacement pattern taken up by the basilar membrane will be complex indeed. Hair cells at different points among the basilar membrane will be stimulated and each to a different extent. The combination of all the stimulations will be interpreted by the brain as a variety of pitches which, taken all together, will make up the "quality" of a sound. Thus, a piano and a violin sounding the same tone will produce effects that are clearly different. Each will set up a number of sets of vibrations at varying frequencies, even though the dominant frequency will be that of the tone being sounded. Because a violin and piano have radically different shapes, each will resonate in different fashion to these varying frequencies, so one may reinforce frequency A more than frequency B and the other may reverse matters.
In musical sounds, the differing frequencies of the sound waves set up bear simple numerical relationships among themselves. In nonmusical sounds, the various frequencies are more randomly distributed. The basilar membrane of the cochlea can undergo displacements in response to any sound, musical or not. However, we interpret the simple numerical relationships among simultaneous frequencies as "chords" and "harmonies" and find them pleasant, whereas the frequencies not in simple numerical relationships are "discords" or "noise" and are often found unpleasant.
The delicacy with which we can distinguish pitch and the total range of pitch we can hear depend on the number of hair cells the cochlea can hold and therefore on the length of the organ of Corti. It is clearly advantageous, then, to have the cochlea as long as possible; the human cochlea is one and a half inches long. Or at least it would be that length if it were straight; by being coiled into a spiral (forming two and a half turns), it takes up less room without sacrifice of length.
The human ear can detect sound from frequencies as low as 16 cycles per second (with a wavelength of about 70 feet) to frequencies as high as 25,000 cycles per second (wavelength, about half an inch). In music each doubling of frequency is
considered an octave ("eight" L, because in the diatonic scale each octave is divided into seven different tones, the eighth tone starting a new octave) and, therefore, the ear has a range of a little over ten octaves. The width of this range may be emphasized by the reminder that the full stretch of notes on the piano extends over only yJa octaves.
The ear is not equally sensitive to all pitches. It is most sensitive to the range from about 1000 to 4000 cycles per second. This range corresponds to the stretch from the note C two octaves above middle C to the note C that is two octaves higher still. With age, the range of pitch shrinks, particularly at the shrill end; children can easily hear high-pitched sounds that to an adult are simply silence. It is estimated that after the age of forty, the upper limit of the range decreases by about 13 cycles per month.
There are sound waves of frequencies outside the range we can hear, of course. Those with frequencies too high to hear are ultrasonic waves ("beyond sound" L), and those with freqtiencies too low are subsonic waves ("below sound" L).° In general, larger animals, with larger sound-producing and sound-sensing organs, can produce and hear deeper sounds than can smaller animals. The smaller animals, in turn, can produce and hear shriller sounds. The trumpet of an elephant and the squeak of a mouse represent reasonable extremes.
While few animals are sensitive to the wide range of pitch we are sensitive to, we are comparatively large creatures. It is easy to find among smaller creatures examples of animals that can readily hear sounds in the ultrasonic reaches. The songs of many birds have their ultrasonic components, and we miss much of the beauty for not hearing these. The squeaking of mice and bats is also rich in ultrasonics, and in the latter case, at least, these have an important function which I shall describe below. Cats and dogs can hear shrill sounds we cannot. The cat will
" These days the adjective "supersonic" f "above sound" L) is much used. This does not refer to a range of sound frequencies, but to a velocity that is greater than the speed of sound.
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detect a mouse's high-pitched squeak which to us may be a faint sound or nothing at all, and dogs can detect easily the ultrasonic vibrations of the silent "dog-whistles" that are silent only to ourselves.
ECHOLOCATION
In hearing we not only detect a sound but to a certain extent we also determine the direction from which it comes. That we can do so is largely thanks to the fact that we have two ears, the existence of which is not a matter of symmetrical esthetics alone. A sound coming from one side reaches the ear on that side a little sooner than it does the ear on the other. Furthermore, the head itself forms a barrier that sound must pass before reaching the more distant ear; the wave may be slightly weakened by the time it gets there. The brain is capable of analyzing such minute differences in timing and intensity (and the experience of living and of years of trying to locate sounds in this manner and observing our own success sharpens its ability to do so) and judging from that the direction of the sound.
Our ability to judge the direction of sound is not equal throughout the range of pitch we can hear. Any wave-form reacts differently toward obstacles according to whether these are larger or smaller than its own wavelength. Objects larger than a wavelength of the wave-form striking them tend to reflect the waveform. Objects that are smaller do not; instead the wave-form tends to go around it. The smaller the object in comparison to the wavelength, the less of an obstacle it is and the more easily it is "gone around."
The wavelengths of the ordinary sounds about us is in the neighborhood of a yard, which means that sound can travel around comers and about the average household obstacle. (It will, however, be reflected by large walls and, notoriously, by mountainsides, to produce echoes.) The deeper the sound the more easily does it move around the head without trouble and
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the less is it weakened before reaching the far ear. One method of locating a sound is therefore denied us. The effect is to be seen in the way the majestic swell of the organ in its lower registers seems to "come from all about us,** and thereby to be the more impressive. On the other hand, a particularly shrill note with a wavelength of an inch or so finds the head too much of a barrier, and possibly the far ear does not get enough of the sound to make a judgment. Certainly it is difficult to locate a cricket in a room from the sound of its shrill chirp.
The use of both ears, binaural hearing (bin-aw'rul; "two ears" L), does not merely help in locating a sound but also aids sensitivity. The two ears seem to add their responses, so that a sound heard by both seems louder than when heard by only one. Differences in pitch are also more easily distinguished with both ears open than with one covered.
Echoes themselves can be used for location of the presence of a barrier. Thus, when driving along a line of irregularly parked cars, we can, if we listen, easily tell the difference in the engine sound of our own car as we pass parked cars and the engine sound as we pass empty parking places. In the former case the engine sound has its echo added, and there would be no difficulty in locating, through the contrast, an unoccupied parking place with our eyes closed. Unfortunately, we could not tell whether that unoccupied parking place contained a fireplug or not. An automobile is large enough to reflect the wavelengths of some of our engine noises but a fireplug is not. To detect objects smaller than a car would require sound waves of shorter wavelength and higher frequency. The shorter the wavelength and the higher the frequency, the smaller the object we can detect by tbe echoes to which it gives rise. Obviously, ultrasonic sound would be more efficient in this respect than ordinary sound.
Bats, for example, have long puzzled biologists by their ability to avoid obstacles in flight and to catch insects on the wing at night, even after having been blinded. Deafening bats destroys this ability, and this was puzzling indeed at first. (Can a bat see
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with its ears? The answer is yes, in a way it can.) It is now known that a flying bat emits a continuous series of ultrasonic squeaks, with frequencies of 40,000 to 80,000 cycles per second ~(and with resulting wavelengths of from 1/3 of an inch down to 1/6 of an inch). A twig or an insect will tend to reflect such short wavelengths, and the bat, whose squeaks are of excessively short duration, will catch the faint echo between squeaks. From the time lapse between squeak and echo, from the direction of the echo and the extent of the echo's weakening, it can apparently tell whether an object is a twig or an insect and exactly where the object is. It can then guide its flight either with an intention of avoiding or intersecting, as the case may be. This is called echolocation, and we should not be surprised that bats have such large ears in relation to their overall size.
Dolphins apparently have a highly developed sense of echo-location, too, though they make use of generally lower sounds, since they require reflection from generally larger objects. (Dolphins eat fish and not insects.) It is by echolocation that dolphins can detect the presence of food and move toward it unerringly even in murky water and at night, when the sense of sight is inadequate.
Man has more of this power of echolocation than he usually suspects. I have already mentioned the ability to locate an empty parking spot, which you may try for yourself. That we do not depend on such devices more than we do is simply because our reliance on sight is such that we ordinarily ignore the help of the ear in the precise location of objects, at least consciously.
Nevertheless, a blindfolded man walking along a corridor can learn to stop before he reaches a blocking screen, as a result of hearing the change in the echoes of his footsteps. He can do this even when he is not quite aware of what it is he is sensing. He may then interpret matters as "I just had a feeling —" Blind men, forced into a greater reliance on hearing, develop abilities in this respect which seem amazing but are merely the result of exploiting powers that have been there all the time.
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Mechanically, man has learned to use ultrasonic waves for echolocation (in the precise manner of bats) in a device called sonar, which is an abbreviation for "sound navigation and ranging." Sonar is used for detecting objects such as submarines, schools of fish, and bottom features in the ocean. In the open air men now make use of microwaves (a form of light waves with wavelengths in the range of those of ultrasonic sound) for the same purpose. Echolocation by microwave is generally referred to as radar, an abbreviation for "radio detection and ranging." (Microwaves are sometimes considered very short radio waves, you see.)
THE VESTIBULAR SENSE
The acoustic nerve, which leads from the cochlea, has a branch leading to the other half of the contents of the internal ear, the utricle and its outgrowths, introduced on page 248. It is time to consider in detail their function. In its simplest form, the utricle may be viewed as a hollow sphere filled with fluid and lined along its inner surface with hair cells. (The structure is similar to the saccule and its outgrowths.) Within the sphere is a bit of calcium carbonate which, thanks to gravity, remains at the bottom of the sphere and stimulates the hair cells there.
Imagine a fish swimming at perfect right angles to the pull of gravity — in a perfectly horizontal line, and leaning neither to one side nor the other. The bit of calcium continues to remain at the bottom of the sphere, and it is the stimulation of those particular hair cells which is interpreted by the nervous system as signifying "normal posture." If the fish's direction of swimming tilts upward, the sphere changes position and the bit of calcium carbonate settles to the new bottom under the pull of gravity, stimulating hair cells that are farther back than the normal-posture ones. If the direction of swim tilts downward, hair cells in front of the normal-posture ones are stimulated. Again there is a shift to the right with a rightward tilt and to the left with a leftward tilt. When the fish is upside-down, the calcium carbonate
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OUR E A H S
is stimulating hair cells that are removed by 180 degrees from the normal-posture ones.
In all these cases, the fish can automatically right itself by moving in such a way as to bring the bit of calcium carbonate back to the normal-posture hair cells. The function of the utricle, we observe, is to maintain the normal posture. To us that would be an upright standing position, so a utricle used for this purpose may be called a statocyst ("standing-pouch" G) and the bit of calcium carbonate is the statolith ("standing-stone" G).
This function can be shown dramatically in crustaceans. The statocysts in such creatures open to the outside world through narrow apertures, and the statoliths are not bits of calcium carbonate but are, rather, sand particles the creature actually places within the statocysts. When the crustacean molts, those bits of sand are lost and must be replaced. One experimenter removed all sand from a tank and substituted iron filings. The shrimp with which he was experimenting innocently introduced iron filings into the statocyst. Once this was done, a magnet held above the shrimp lifted the filings against the pull of gravity and caused them to stimulate the uppermost hair cells instead of the lowermost. In response the animal promptly stood on its head, so that the lowermost hair cells might be stimulated by the "upward-falling" filings.
Because the statocyst is located in the internal ear, it is more commonly, though less appropriately, called the otocyst (oh'toh-sist; "ear pouch" G). The material within, if present in relatively large particles, is called otoltths ("earstone" G), and if present in fine particles is called otoconia (oh'toh-koh'nee-uh; "ear-dust" G). Otoconia persist in the utricle of the land vertebrates. The vestibular sense made possible by the utricle is somewhat reminiscent of the proprioceptive senses (see p. 221). However, where the proprioceptive senses tell us the position of one part of the body with relation to another, the vestibular sense tells us the position of the body as a whole with respect to its environment, especially with regard to the direction of the pull of gravity.
267
A cat can right itself when falling and land on its foot, e\en though it was dropped feet up- It does this L>\ automatically altering the position of its head into the upright, being guided by the position of its otoconia. This in turn brings about movements in the rest of its body designed to bring it into line with the new position of the head. Down it comes, feet first every time. Nor are \ve ourselves depri\ed. \\'e have no difficulty in telling whether we are standing upright, upside-down, or tilted in any possible direction, even with our eyes closed and even when floating in water. A swimmer who dives into the water can come up headfirst without trouble and without having to figure out his position consciously.
But the utricle is not all there is to the vestibular sense. Attached to the utricle are three tubes that start and end there, each bending in a semicircle so they are called semicircular canals. Each semicircular canal is filled with fluid and is set in an appropriate tunnel within the bone of the skull but is separated from the bone by a thin layer of the fluid. The individual semicircular canals are arranged as follows. Two are located in a vertical plane (if viewed in a standing man) but at right angles to each other, one directed forward and outward, the other backward and outward. The third semicircular canal lies in a horizontal plane. The net result is that each semicircular canal lies in a plane at right angles to those of the other two. You can see the arrangement if you look at a corner of the room where two walls meet the floor. Imagine the curve of one canal following the plane of one wall, that of a seeond canal following the plane of the other wall, and that of the third canal following the plane of the floor. One end of each canal, where it Joins the utricle, swells out to form an ampulla (am-pul'uh; "little vase" G, because of its shape). Within each ampulla is a small elevated region called a crista ("crest" L), which contains the sensitive hair cells.
The semicircular canals do not react to the body's position with respect to gravity; they react to a change in the body's position. If you should turn your head right or left or tilt it up or
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down, or in any combination of these movements, the fluid within one or more of the semicircular canals moves because of inertia. There is thus a flow in the direction opposite to the head's motion. (If your car makes a right turn you are pressed against the left, and vice versa.) By receiving impulses from the various stimulated hair cells as a result of this inertial flow of liquid and by noting which were stimulated and by how much, the mind can judge the nature of the motion of the head."
The semicircular canals judge not motion itself, then, but change of motion. It is acceleration or deceleration that makes fluid move inertially. (In a car at steady speed, you sit comfortably in your seat. But when the car speeds up, you are pressed backward, and when the car slows down, you are pressed forward.) This means that stopping motion is as effective as starting motion in stimulating the semicircular canals. This becomes very noticeable if we spin about as rapidly as we can and continue it long enough to allow the fluid within the semicircular canals to overcome inertia and to turn with us. Now if we stop suddenly, the fluid, thanks to its inertia, keeps on moving and stimulates the hair cells strongly. We interpret this as signifying that there is relative motion between ourselves and our surroundings. Since we know we are standing still, the only conclusion is that the surroundings are moving. The room seems to spin about us, we are dizzy, and in many cases can do nothing but fall to the ground and hold desperately to the floor until the fluid in our semicircular canals settles down and the world steadies itself.
The steady rocking motion of a ship also stimulates the semicircular canals, and to those who are not used to this overstimu-lation the result often is seasickness, which is an extremely unpleasant, though not really fatal affliction.
* The lampreys, among the most primitive living vertebrates, have only two semicircular canals. Their prefish ancestors were bottom-dwellers who had to contend with motions left and right, forward and backward, but not up and down. They lived a two-dimensional life. The fish developed the third canal for the up-down dimension as well, and all vertebrates since — including ourselves, of course — have had a three-dimensional vestibular sense.
12
OUR EYES
LIGHT
The earth is bathed in light from the sun and one could scarcely think of a more important single fact than that. The radiation of the sun (of which light itself is an important but not the only component) keeps the surface of the earth at a temperature that makes life as we know it possible. The energy of sunlight, in the early dawn of the earth, may have brought about the specific chemical reactions in the ocean that led to the formation of life. And, in a sense, sunlight daily creates life even now. It is the energy source used by green plants to convert atmospheric carbon dioxide into carbohydrates and other tissue components. Since all the animal kingdom, including ourselves, feeds directly or indirectly on green plants, sunlight supports us all. Again, the animal kingdom, and man in particular, has grown adapted to the detection of light. That detection bas become so essential to us as a means of sensing and interpreting our environment that blindness is a major affliction, and even fuzzy vision is a serious handicap.
Light has also had a profound influence on the development of science. For the last three centuries the question of the nature of light and the significance of its properties has remained a cni-cial matter of dispute among physicists. The two chief views concerning the nature of light were first propounded in some
27O THE HUMAN BRAIN
detail by 17th-century physicists. The Englishman Isaac Newton believed that light consisted of speeding particles; the Dutchman Christian Huygens believed that it was a wave-form. Central to the dispute was the fact that light traveled in straight lines and cast sharp shadows. Speeding particles (if unaffected by gravity) would naturally move in straight lines, whereas all man's experience with water waves and sound waves showed that wave-forms would not, but would bend about obstacles. For a century and a half, then, the particle theory held fast.
In 1801 the English scientist Thomas Young demonstrated that light showed the property of interference. That is to say, two rays of light could be projected in such a way that when they fell upon a screen together areas of darkness were formed. Particles could not account for this, but waves could — because the wave of one ray might be moving upward while the wave of the other was moving downward, and the two effects would cancel.
The wave theory was quickly made consistent with the straight-line travel of light, since Young also worked out the wavelength of light. As I have said in the previous chapter, the shorter the wavelength the less a wave-form is capable of moving about obstacles, and the more it must move in a straight line and cast shadows. The very shortest wavelengths of audible sound are in the neighborhood of half an inch, and they already show considerable powers of straight-line travel. Imagine, then, what light must be able to do in this respect when we consider that a typical light wave has a wavelength of about a fifty-thousandth of an inch. Light is much more efficient than even the most ultrasonic of life-produced sound in echolocation. We may be able to detect the position of an object by the sound it makes, but we do so only fairly well. When we see an object, on the contrary, we are quite certain that we know exactly where it is. "Seeing is believing," we say, and the height of skepticism is "to doubt the evidence of one's own eyes."
Light waves contain far mere energy than do the sound waves we ordinarily encounter; enough energy, as a matter of fact, to
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bring about chemical changes in many substances. It is quite feasible for living organisms to detect the presence of light by the presence or absence of such chemical changes and to respond accordingly. For the purpose it is not even necessary to develop an elaborate light-detecting organ. Plants, for instance, climb toward the light, or bend toward it, without any trace of such an organ. A response to light is clearly useful. All green plants must grow toward the light if they are to make use of its energy. Water animals can find the surface layers of the sea by moving toward the light. On land, light means warmth and animals may seek it or avoid it depending on the season of the year, the time of day, and other factors.
Detecting light by its chemical effect can, however, be dangerous as well as useful. In living tissue, with its delicate balance of complex and fragile interacting compounds, random changes induced by light can be ruinous. It proved evolutionarily useful to concentrate a chemical particularly sensitive to light in one spot. Because of its individual sensitivity, such a chemical would react to a low intensity of light, one that would not damage tissue generally, Furthermore, its location in a certain spot would enable the remainder of the organism's surface to be shielded from light altogether.
(In order for any substance to be affected by light to the point of chemical change, it must first absorb the light. Generally it will absorb some wavelengths of light to a greater extent than others, and the light it reflects or transmits will then be weighted in favor of the wavelengths it does not absorb. But we sense different wavelengths, as I shall explain later in the chapter, as different colors, so when we see the light-sensitive substance by the light it transmits or reflects we see it as colored. For this reason the light-sensitive compounds in organisms are commonly referred to as pigments, a word reserved for colored substances, and specifically as visual pigments.)
Even one-celled animals may have light-sensitive areas, but the true elaboration of course comes in multicellular animals, in
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which discrete organs — eyes — are devoted to photoreception. (The prefix "photo" is from the Greek word for "light.")
The simplest photoreceptors can do no more than detect light or not detect it. Nevertheless, even when an organism is limited to this detection it has a useful tool. It can move either toward or away from the light sensed. Furthermore, if the level of stimulation suddenly falls, the obvious inteqiretation is that something has passed between the photoreceptor and the light. Flight could be a logical response, since the "something," after all, might very well he an enemy.
The more sensitive a photoreceptor can be made the better, and one method of increasing the sensitivity is to increase the amount of light falling upon the visual pigment. A way of doing this depends upon the fact that light tloes not necessarily travel in a straight line under all conditions. Whenever light passes obliquely from one medium to another it is bent or refracted ("bent back" L). If the surface between media is flat, all the light entering it bends as a unit.0 If the surface is curved, things are rather more complicated. Should light pass from air into water across a surface that is more or less spherical, the rays tend to bend in the direction of the center of the sphere, no matter where they strike. All the light rays converge therefore and are eventually gathered together into a focus ("fireplace" L, since that is where light is gathered together, so to speak, in a household).
To concentrate light, organisms use not water itself but a transparent object that is largely water. In land animals it is shaped like a lentil seed, which in Latin is called lens and which lent its name to the shape. A lens is a kind of flattened sphere that does the work but economizes on room. The lens acts to concentrate light; all the light that falls on its relatively broad width is brought into the compass of a narrow spot. A child can use a lens to set paper on fire, whereas unconcentrated sunlight would be helpless to do so. In the same way a particular photo-
" This is strictly so only if alt the light is of the same wavelength. Where it is not there is another important effect (see p. 295).
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receptor could respond to feeble light which, in the absence of lens-concentration, would leave it unaffected.
Since light, left to itself, travels primarily in straight lines, a photoreceptor — whether equipped with a lens or not — will sense light only from the direction it faces. To sense light in other directions a creature must turn, or else it must be supplied with photoreceptors pointed in a number of directions. The latter alternative has much to recommend it since it saves the time required to turn, and even a fraction of a second may be important in the eternal battle to obtain food and avoid enemies.
The development of multiple photoreceptors reaches its climax in insects. The eyes of a fly are not single organs. Each is a compound eye made up of thousands of photoreceptors, each of which is set at a slightly different angle. A fly without moving can be conscious of changes in light intensity at almost any angle, which is why it is so difficult to catch one by surprise while bringing it the gift of a flyswatter. Each photoreceptor of the compound eye registers only "light" or "dark" but their numbers enable something more to be done. If an object lies between the compound eye and the light, the insect can obtain a rough estimate of the object's size and shape by the number and distribution of those photoreceptors that register "dark." A kind of rough mosaic picture is built up of the object. Furthermore, if the object moves, individual photoreceptors go dark progressively in the direction of its motion, and others light up progressively as it leaves. In this way, the insect can obtain an idea of the direction and velocity of a movement.
The vertebrates have adopted a different system. Use is made of large individual eyes that concentrate light on an area of photosensitive cells. Each cell is individually capable of registering light or dark. The individual photoreceptors are cell-sized and microscopic; and not, as in insects, large enough to be seen by the naked eye. The vertebrate mosaic of sight is fine indeed.
Suppose that you try to draw a picture of a man's face on a sheet of paper using black dots after the fashion of a newspaper
CRYSTALLINE LENS
PUPIL
FOVEA CENTRALIS AND
MACULEA LUTEA
CHOROID
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^Jr OPTIC CHIASMA
EYES AND
OPTIC NERVES
FROM ABOVE
photograph (look at one under a magnifying glass and you will see what I mean). If you use large dots for the purpose, you can't get much detail into the picture. If you use smaller dots (for a picture of the same size) you can make out more detail; still smaller dots, still greater detail.
The "dots" used by insects are the size of the individual facets of their compound eyes; the dots used by ourselves are the size of cells. We can therefore see much more detail than an insect can; our vision is much more acute. In the space which a honeybee might cover with one dot, either light or dark — and that is all the information it would have — we could squeeze in some 10,000 dots in a possibly intricate pattern of light and dark that could yield a great deal of information.
The use of an eye with cell-sized photoreceptors offers such advantages that it has actually been devised by a number of quite unrelated groups of animals. In particular, certain groups of mol-lusks developed an eye quite independently of the development going on in vertebrates, and ended almost in the same place. The eye of the squid, though possessing a completely different history than ours does, resembles ours closely, part for part.
THE EYEBALL
The human eye, which is just about an inch in diameter, is very nearly a sphere in shape, so the expression eyeball for the eye as a physical structure is quite apt. About five sixths of the eyeball is enclosed by a strong, fibrous outermost layer called the sclera (sklee'ruh; "hard" G). It is white, and portions of it are visible in that part of the eye to be seen between the eyelids. This is referred to as "the white of the eye."
In the front of the eye, facing the outside world directly, is a section about half an inch in diameter which is transparent. It is the cornea. (This word is from the same root as the word "horn," and since thin layers of horn are semitransparent, and since horn and cornea are both modified skin, the name is not as
276 THE HUMAN BRAIN
farfetched as it might seem,) The cornea does not really complete the sphere of the eyeball smoothly. Its curvature is sharper than is that of the sclera and it bulges outward from the eye's smooth curve, like a portion of a small sphere set into the side of a larger one. If you close your eyes lightly, place your finger upon the eyelid, and move your eye, you will definitely feel the bulge of the cornea.
A layer of dark tissue, lining the inner surface of the sclera, continues the smooth curve of the eyeball and extends out into the cavity formed by the cornea's bulge, almost closing the transparent gap. This is the choroid (koh'roid; "membrane" G) and it is well supplied with blood vessels, some of which occasionally show through the white of the sclera. The portion of the choroid visible under the cornea contains the dark pigment, melanin, which is responsible for the brown or black color of hair, and for the swarthiness of skin. In most human beings, there is enough melanin in this portion of the choroid to give it a brown color. Among fair-skinned individuals with less-than-average capacity for forming melanin, the color is lighter. If the spots of pigment are sparse enough, they do not absorb light so much as scatter it. Light of short wavelength fas, for example, blue) is more easily scattered than light of long wavelength (such as red), so that the visible choroid usually appears blue or blue-green under these conditions, viewed as it is by the light it scatters. At birth, babies' eyes are always blue but as pigment is formed in increasing quantities during the time of infancy, most sets of eyes gradually turn brown. Among albinos, incapable of forming melanin altogether, the choroid contains no pigment and the blood vessels are clearly seen, giving the choroid a distinctly reddish appearance.
The different colors found among individuals in this portion of the eye give it the name iris ("rainbow" G). We are particularly conscious of this color, and when we speak of "brown eyes" or "blue eyes" we are referring to the color of the iris and not of the whole eye, of course. The function of the iris is to screen the light entering the interior of the eyeball. Naturally, the more
O V It E Y
pigmented the iris, the more efficiently it performs this function. The evolutionary development of blue eyes took place in northern countries, where sunlight is characteristically weak and where imperfect shading could even he useful in increasing the eye's sensitivity. An albino's eyes are unusually sensitive to light he-cause of the lack of shading, and he must avoid hright lights.
To allow for changes in the intensity of outside illumination the iri.s is equipped to extend or contract the area it covers by means of the tiny muscle fibers it contains. In bright light, the fibers relax and the iris covers almost the entire area under the cornea. A liny round opening is left through which light can enter the eyeball, and this opening is the jmpil ("doll" L, because of the tiny image of oneself one can see reflected there). In dim light, the fibers tighten and the iris draws back, so that the pupil enlarges and allows more light to enter the eyeball.
The pupil is the opening through which we actually see, and this is evident even to folk-wisdom, which refers to it as "the apple of the eye" and uses it as a synonym for something carefully loved and guarded. It is partly through the variation in its size that we adapt to a specific level of light. Entering a darkened movie theater from the outer sunlight leaves us blind at first. If we wait a few minutes, the pupil expands and our vision improves greatly as more light pours in. Conversely, if we stumble into the bathroom at night and turn on the light we find ourselves momentarily pained by the brilliance. After a few moments of peering through narrowed eyelids, the pupil decreases in size and we are comfortable again. At its smallest the pupil has a diameter of about 1.5 millimeters (about 1/16 inch), at its widest about 8 or 9 millimeters (over 13 inch). The diameter increases sixfold, and since the light-gathering power depends on the area — which varies as the square of the diameter — the pupil at maximum opening admits nearly forty times as much light as at minimum opening. (Our pupil retains a circular shape as it grows larger and smaller. This is not true of some other animals. In the cat, to cite the most familiar case, the pupil is round in the dark,
2/8 THE HUMAN BRAIN
but with increasing light it narrows from side to side only, becoming nothing more than a vertical slit in bright light.}
The eye is carefully protected from mechanical irritation as well as from the effects of too much light. It is equipped with eyelids which close rapidly at the slightest hint of danger to the eyes. So rapid is this movement that "quick as a wink" is a common phrase to signify speed, and the German word for "an instant" is ein Augenblick ("an eyewink"). Nor is the movement of the eyelid itself a source of irritation. For one thing, there is a delicate membrane covering the exposed portion of the eyeball and the inner surface of the eyelid. This is the conjunctiva (kon'-junk-ty'vuh; "connective" L, because it connects the eyeball and the eyelids). The conjunctiva is kept moist by the secretions ("tears") of the tear glands, a name that is Latinized to lacrimal glands ("tear" L). These are located just under the bone forming the upper and outer part of the eye socket.
When the eyelid closes, conjunctiva slides along conjunctiva with a thin, lubricating layer of fluid between. In order to keep the eye's surface moist and flexible, the eyelid closes periodically, moving fluid over the exposed portion of the eye, despite the fact that danger may not be present. We are so used to this periodic blinking, even when not consciously aware of it, that we are made uneasy by an unwinking stare. The fact that snakes do not have eyelids and therefore have just such an unwinking stare is one factor in their appearance of malevolence.
Some animals have a third eyelid in the form of a transparent membrane that can be quickly drawn across the eye, usually in a horizontal sweep from the inner comer of the eye (the inner canthus) near the nose. This is the nictitating membrane ("wink" L) and cleans the eye without introducing a dangerous, even if short, period of complete blindness. Man does not possess a functional nictitating membrane, but a remnant of it is to be found in the inner canthus.
Tears also serve the purpose of washing out foreign matter that gets onto the eye's surface, The eye is protected against such
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foreign matter not only by the eyelid itself but by the eyelashes that rim the lids and permit sight while maintaining a protective (but discontinuous) barrier across the opening. Thus, we automatically squint our eyes when the wind stirs up dust. There also are the eyebrows, which protect the overhang of the forehead, entangling raindrops and insects.
Still, foreign matter will invade the eye occasionally. Sometimes an eyelash will get in so that the protective device itself can be a source of trouble. In response to such invasion (which can be exquisitely uncomfortable), the lacrimal glands secrete their tears at an increased rate and the eyes "water." Eyes will also water in response to the irritation of smoke, chemicals (as in the case of the well-known "tear gas"), strong wind, or even strong light. Ordinarily tears are carried off by the thin lacrimal ducts placed at the inner canthus. The fluid is then discharged into the nasal cavity. Usually not enough of it is disposed of in this fashion to be noticed. When the lacrimal ducts grow in-
o
flamed during infection, tear outflow is cut down, and we notice the lack of the duct action easily enough since watering eyes make up one of the unpleasant symptoms of a cold.
In response to strong emotions the lacrimal glands are particularly active, and secrete tears past the capacity of the lacrimal ducts, even at their best, to dispose of them. In such cases, the tears will collect and overflow the lower lids so that we weep with rage, with joy, with frustration, or with grief, as the case maybe. The escape of tears into the nasal cavity does become noticeable under these conditions, too, and it is common to find one must blow one's nose after weeping.) Tears are salt, as are all body fluids, and also contain a protein called lysozyme, which has the ability to kill bacteria and thus lend tears a disinfecting quality.
Despite all the protection offered them, the eyes, of necessity, are unusually open to irritation and infection, and the inflammation of the conjunctiva that results is conjunctivitis. The engorged blood vessels, unusually visible through the sclera, give the eye a "bloodshot" appearance. Newborn babies are liable to develop
28o THE HUMAN BRAIN
conjunctivitis because of infections gained during their passage through the genital canal. This, however, is controlled and prevented from bringing about serious trouble by the routine treatment of their eyes with antibiotics or with dilute silver nitrate solution.
A serious form of conjunctivitis, caused by a virus, is trachoma (tra-koh'muh; "rough" G), so called because the scars formed on the eyeball give it a roughened appearance. The scars on the cornea can be bad enough to blind a sufferer of the disease. Since trachoma is particularly common in the Middle East, this may possibly account for the number of blind beggars featured in the stories of the Arabian Nights.
The fact that we have two eyes is part of our bilateral symmetry, as is the fact that we have two ears, two arms, and two legs. The existence of two eyes is useful in that the loss of vision in one eye does not prevent an individual from leading a reasonably normal-sighted life. However, the second eye is more than a spare.
In most animals, the two eyes have separate fields of vision and nothing, or almost nothing, that one of them sees can be seen by the other. This is useful if a creature must be continually on the outlook for enemies and must seemingly look in all directions at once. Among the primates, though, the two eyes are brought forward to the front of the head, so that the fields of vision overlap almost entirely. What we see with one eye is just about what we see with the other. By so narrowing the field of vision, we look at one thing and see it clearly. Moreover, we gain importantly in depth perception.
We can judge the comparative distances of objects we see in a number of ways, some of which depend upon experience. Knowing the true size of something, say, we can judge its distance from its apparent size. If we don't know its true size we may compare it with nearby objects of known size. We can judge by the quantity of haze that obscures it, by the convergence of parallel lines reaching out toward it, and so on. All this will work for one eye as well as for two, so that depth perception with one eye
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28l
is possible." Nevertheless, we have but to close one eye to see that, in comparison with two-eyed vision, one-eyed vision tends to be flat.
With two eyes, you see, the phenomenon of parallax is introduced. We see a tree with the left eye against a certain spot on the far horizon. We see the same tree at the same time with the right eye against a different spot on the far horizon. (Try holding a pencil a foot before your eyes and view it while closing first one eye then the other without moving your head. You will see it shift positions against the background.) The closer an object to the eye, the greater its shift in position with change from one view to the other. The field of vision of the left eye therefore differs from that of the right in the relative positions of the various objects the field contains. The fusion of the two fields enables us to judge comparative distance by noting (quite automatically and without conscious effort) the degrees of difference. This form of depth perception is stereoscopic vision ("solid-seeing" G), because it makes possible the perceiving of a solid as a solid, in depth as well as in height and breadth and not merely as a flat projection.8 "
The fixing of the eyes upon a single field of vision does not obviate the necessity of seeing in all directions. One way of making up for the loss of area of the field of vision is to be able to turn the neck with agility. The owl, whose stereoscopic eyes arc fixed in position, can turn its neck in either direction through almost 180 degrees so that it can look almost directly backward.
* By cleverly altering backgrounds to take advantage of the assumptions we are continually making, we can be tricked into coming to false conclusions as to shapes, sizes, and distances, and thi.s is the explanation of many of the "oplical illusions" with which we all amuse ourselves at one time or another.
00 In the days before movies a popular evening pastime was to look ;it stereoscope slides. These consisted of pairs of pictures of the same scene taken from slightly different angles, representing the view as it would be seen by a left eye alone and also by a right eye. Looking at these through a device that enables the eyes to fuse the pictures into one scene caused the view to spring into a pseudo-three-dimensional reality. In the 3-D craze that hit the movies in the i9so's, two pictures were taken in this same left-eye-right-eye manner and the two were viewed separately by either eye when special spectacles of oppositely polarized film were put on.
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Our own less flexible neck will not permit a turn of more than 90 degrees, but, on the other hand, we can turn our eyeballs through a considerable angle. The human eyeball is outfitted with three pairs of muscles for this purpose. One pair turns it right or left; one pair up or down; and one pair rotates it somewhat. As a result, a reasonable extension of the field of vision is made possible by a flicker of movement taking less time and effort than moving the entire head.
EYE MUSCLES
The restriction of field of vision makes it possible for a man to be surprised from behind ("Do I have eyes in the back of my head?" is the plaintive cry), but to the developing primates in the trees, stereoscopic vision, which made it possible to judge the distance of branches with new precision, was well worth the risk of blindness toward the rear. In nonstereoscopic vision there is no reason why the motion of the eyes might not be independent of each other. This is so in the chameleon, to cite one case, and the separate movements of its eyes are amusing to watch. In stereoscopic eyes such as our own, however, the two eyeballs must move in unison if we are to keep a single field in view.
Occasionally a person is to be found who has an eyeball under defective muscular control, so that when one eye is fixed on an object, the other is pointed too far toward the nose ("cross-eyes") or too far away from it ("wall-eyes"). The two conditions are lumped as "squint-eyes," or strabismus (stra-biz'mus; "squint" G). This ruins stereoscopic vision and causes the person to favor one eye over the other, ignoring what is seen with the unfavored eye and causing the latter's visual ability to decline.
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To be sure, the eyes do not, under normal conditions, point in exactly parallel fashion. If both ;ire to orient their pupils in the direction of the same object, they must converge slightly. Usually this convergence is too small to notice, but it becomes more marked for closer objects. If you bring a pencil toward a person's nose, you will see his eyes begin to "cross.'' The extent of the effort involved in such convergence offers another means by which a person can judge distance.
WITHIN THE EYE
Immediately behind the pupil is the lens, sometimes called the crystalline lens, not because it contains crystals, but because, in the older sense of the word "crystalline," it is transparent (see illustration, p. 274). The lens is lens-shaped (of course) and is about a third of an inch in diameter: All around its rim is a ringlike suspensory ligament, which joins it to a portion of the choroid layer immediately behind the iris. That portion of the choroid layer is the ciliary body, and this contains the ciliary muscle.
The lens and the suspensory ligament divide the eyes into two chambers, of which the forward is only one fifth the size of the portion lying behind the lens. The smaller forward chamber contains a watery fluid called the aqueous humor ("water)- fluid" L), which is much like cerebrospinal fluid in composition and circulates in the same way that cerebrospinal fluid does. The aqueous humor leaks into the anterior chamber from nets of capillaries in the ciliary body and out again through a small duct near the point where the iris meets the cornea. This duct is called the canal of Schlemm after the German anatomist Friedrich Schlemm, who described it in 1830.
The portion behind the lens is filled with a clear, jellylike substance, the vitreous humor ("glassy fluid" L), or, since it is not really a fluid, the vitreous body. It is permanent and does not circulate. For all its gelatinous nature, the vitreous body is ordi-
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narily as clear as water. However, small objects finding their way into it are trapped in its jellylike network and can then make themselves visible to us as tiny dots or filaments if we stare at some featureless background. They usually cannot be focused upon but drift away if we try to look at them directly, and so are called "floaters." The Latin medical name muscae volitantes (mus'see vol-ih-tan'teez) sounds formidable but is rather colorful, really, since it means "flying flies." Almost everyone possesses them, and the brain leams to ignore them if the situation is not too extreme. Recent research would make it seem that the floaters are red blood corpuscles that occasionally escape from the tiny capillaries in the retina.
The eye is under an internal fluid pressure designed to keep its spherical shape fairly rigid. This internal pressure is some 177 millimeters of mercury higher than the external air pressure, and this pressure is maintained by the neat balance of aqueous humor inflow-and-outflow. If the canal of Schlemm is for any reason narrowed or plugged — through fibrous ingrowths or inflammation, infection, or the gathering of debris — the aqueous humor cannot escape rapidly enough and the internal pressure begins to rise. This condition is referred to as glaucoma (gloh-koh'muh), for reasons to be described later. If pressure rises high enough, as it only too often does in glaucoma, permanent damage will be done to the optic nerve and blindness will result.
Coating the inner surface of the eyeball is the retina ("net" L, for obscure reasons), and it is the retina that contains the photo-receptors (see illustration, p. 274). Light entering the eye passes through the cornea and aqueous humor, through the opening of the pupil, then through the lens and vitreous humor to the retina. In the process, the rays of light originally falling upon the cornea are refracted, gathered together, and focused at a small point on the retina. The sharper the focus, the clearer and more sensitive the vision, naturally.
The lens, despite what one might ordinarily assume, is not the chief agent of refraction. Light rays are bent twice as much by
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the cornea as by the lens. However, whereas the refractive powers of the cornea are fixed, those of the lens are variable. Thus, the lens is normally rather flat and refracts light comparatively little. The light rays reaching the cornea from a distant object diverge infinitesimally in the process and may be considered to be reaching the cornea as virtually parallel rays. The refractive powers of the comea and the flat lens are sufficient to focus this light upon the retina. As the distance of the point being viewed lessens, the light rays reaching the cornea become increasingly divergent. For distances under twenty feet, the divergence is sufficient to prevent focusing upon the retina without an adjustment somewhere. When that happens, the ciliary muscles contract, and this lessens the tension on the suspensory ligaments. The elasticity of the lens causes it to approach the spherical as far as the ligaments permit, and when the latter relax the lens at once bulges outward. This thickening of the lens curves its surface more sharply and increases its powers of refraction, so that the image of the point being viewed is still cast upon the retina. The closer a point under view, the more the lens is allowed to bulge in order to keep the focus upon the retina. This change of lens curvature is accommodation.
There is a limit, of course, to the degree to which a lens can accommodate. As an object comes nearer and nearer, there comes a point (the near point) where the lens simply cannot bulge any further and where refraction cannot be made sufficient. Vision becomes fuzzy and one must withdraw one's head, or the object, in order to see it. The lens loses elasticity with age and becomes increasingly reluctant to accommodate at all. This means that with the years the near point recedes. An individual finds he must retreat from the telephone book bit by bit in order to read a number and, eventually, may have to retreat so far that he can't read it once he finally has it in focus, because it is too small. A young child with normal vision may be able to focus on objects 4 inches from the eye; a young adult on objects 10 inches away; whereas an aging man may not be able to manage any-
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thing closer than 16 inches. This recession of the near point with age is called presbyopia (prez'bee-oh'pee-uh; "old man's vision" G).
Ideally, the light passing through the cornea and lens should focus right on the retina. It often happens, though, that the eyeball is a bit too deep for this. Light focuses at the proper distance, but the retina is not there. By the time light reaches the retina, it has diverged again somewhat. The eye, in an effort to compensate, allows the lens to remain unaccommodated in order that light may be refracted as little as possible and the focus therefore cast as far back as possible. For distant vision, however, where refraction must be less than for close vision, the lens is helpless. It cannot accommodate less than the "no-accommodation-at-all" that suffices for near vision. An individual with deep eyeballs is therefore nearsighted; he sees close objects clearly and distant objects fuzzily. The condition is more formally referred to as myopia. (my-oh'pee-uh; "shut-vision" G). The name arises out of the fact that in an effort to reduce the fuzziness of distant objects, the myopic individual brings his eyelids together, converting his eyes into a sort of pinhole camera that requires no focusing. However, the amount of light entering the eye is decreased, so it is difficult to see (to say nothing of eyelash interference) and the strain on the eyelid muscles will in the long run bring on headaches.
The opposite condition results when an eyeball is too shallow and consequently the light falls on the retina before it is quite focused. In this case the lens, by accommodating, can introduce an additional bit of light refraction that will force light from a distant source into focus on the retina. Light from a near source, requiring still more refraction, cannot be managed. Such an individual has an unusually far-distant near point. He is far-sighted, seeing distant objects with normal clarity and near ones fuzzily. This is hyperopia (hy'per-oh'pee-uh; "beyond-vision" G). For light passing through the cornea and lens to focus precisely, the cornea and lens must each be smoothly curved. The degree
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of curvature along any meridian (vertical, horizontal, diagonal) must be equal. In actual fact this ideal is never quite met; there are always unevennesses, and, as a result, light does not focus in a point but in a short line. If the line is short enough this is not serious; but if it is long, there is considerable fuzziness of vision in both near and far objects. This is astigmatism ("no point" G). Fortunately such defects in refraction are easily corrected by introducing refraction from without by means of glass lenses. (The use of spectacles was one of the few medical advances made during the Middle Ages.) For myopia, lenses are used to diverge light minimally and push the focus backward; and for hyperopia, lenses are used to converge light minimally and push the focus forward. In astigmatism, lenses of uneven curvature are used to cancel out the uneven curvature of the eye.
The transparency of the cornea and lens is not due to any unusual factor in the composition, despite the fact that they are the only truly transparent solid tissues of the body. They are composed of protein and water and their transparency depends, evidently, on an unusual regularity of molecular structure. They are living parts of the body. The cornea can heal itself, for instance, if it is scratched. The level of life, however, must remain low, since neither tissue may be directly infiltrated by blood vessels — that would ruin their all-important transparency. Yet it is only with blood immediately available that a tissue can go about the business of life in an intensive fashion.
This has its advantages. A cornea can, if properly preserved, maintain its integrity after the death of an organism more easily than it would if it had been accustomed, as tissues generally are, to an elaborate blood supply. A cornea will also "take" if transplanted to another individual, whereas a more actively living tissue would not. This means that a person whose cornea has clouded over as a result of injury or infection but whose eyes are completely functional otherwise may regain his sight through a comeal transplantation.
Transparency is not easy to maintain. Any loss of regularity
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of structure will give rise to opaque regions, and the lens, especially, is subject to the development of opacities. This condition can spread so that the entire lens becomes opaque and useless and vision is lost. The possibility of lens opacity increases with age, and it is the greatest single cause of blindness, accounting for about a quarter of the cases of blindness in the United States. Luckily it is possible to remove the lens and make up for the lost refractive powers by properly designed glasses. Since aged lenses have lost accommodative powers anyway, little is sacrificed beyond the inconvenience of the operation and of having to wear glasses, aud such inconvenience is certainly preferable to blindness.
The opacity within a lens is called a cataract. The ordinary meaning of the word is that of "waterfall," but it is derived from Greek terms meaning "to dash downward" and that need not refer to water only. The lens opacity is like a curtain being drawn downward to obscure the window of the eye. Because the presence of the cataract causes the ordinarily black pupil to become clouded over in a grayish or silvery fashion, the word "glaucoma" ("silvery gray" G) was applied to it in ancient times. When "cataract" came into favor, "glaucoma" was pushed away from the lens condition and came to be applied to another optical disorder (described earlier in the chapter), one to which the word does not truly apply etymologically.
THE RETINA
The retinal coating is about the size and thickness of a postage stamp pasted over the internal surface of the eyeball and covering about four fifths of it. (It sometimes gets detached, bringing about blindness, but techniques now exist for binding it back into position.) The retina consists of a number of layers, and of these the ones farthest toward the light are composed largely of nerve cells and their fibers. Underneath these are the actual photo-receptors, which in the human eye are of two types, the rods
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and the cones, obviously named from their shapes. Under the rods and cones and immediately adjacent to the choroid is a film of pigmented cells that send out projections to insinuate themselves between the rods and the cones. These pigmented cells serve to absorb light and cut down reflection that would otherwise blur the retinal reaction to the light falling on it directly.
In animals adapted to vision in dim light, however, the reverse is desired. In them the retina contains a reflecting layer, the tapetum (ta-pee'tum; "carpet" L), which sends light back and gives the retina a second chance at it. Clarity of vision is sacrificed to sensitivity of detection. Some light, even so, escapes the retina after reflection has allowed that tissue a second chance, and this escaping light emerges from the widespread pupils. It is why cats' eyes (tapetum-equipped) gleam eerily in the dark. They would not do so if it were truly dark, because they do not manufacture light. The human eye, needless to say, does not have a tapetum. It sacrifices sensitivity to clarity.
The arrangement of layers in the retina is such that approaching light must in general first strike the layers of nerve cells and pass through them in order to reach the rods and cones. This seems inefficient, but things are not quite that bad in the human eye. At the point of the retina lying directly behind the lens and upon which the light focuses, there is a yellow spot (yellow because of the presence of a pigment) called the macula, lutea (mak'yoo-luh lyoo'tee-uh; "yellow spot" L); see illustration, page 274. In it the photoreceptors are very closely packed, and vision is most acute there.
In order for us to see two separate objects actually as two and ijot have them blur together into one object (and this is what is meant by visual acuity), the light from the two objects would have to fall upon two separate photoreceptors with at least one unstimulated photoreceptor in between. It follows that the more closely packed the photoreceptors, the closer two objects may be and yet have this happen. In the macula lutea the photoreceptors are crowded together so compactly that at ordinary reading
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distance a person with normal vision could see two dots as two dots when separated by only a tenth of a millimeter.
Furthermore, in the very center of the macula lutea there is- a small depression called the fovea centralis (foh'vee-uh sen-tray'lis; "central pit" L) which is right where light focuses. The reason the spot is depressed is that the nerve layers above the photo-receptors are thinned out to almost nothing so that light hits the photoreceptors directly. This situation is most highly developed in the primates. This is one of the reasons why the primate Order, including ourselves, has to such a large extent sacrificed smell and even hearing to the sense of sight. The very excellence of the sense of sight that we have evolved has made it tempting to do so.
Naturally, the retina outside the fovea is not left unused. Light strikes it and the brain responds to that. When we are looking at an object we are also conscious of other objects about it (peripheral vision). We cannot make out small details in peripheral vision, but we can make out shapes and colors. In particular, we can detect motion, and it is important even for humans to see "out of the corner of the eye." In this age of automobiles, many a life has been saved by the detection of motion to one side; license examiners routinely test one's ability to do so by waving pencils to one side while having the applicant stare straight ahead. The loss of peripheral vision (popularly called tunnel vision because one can then only see directly forward) would make one a dangerous person behind the wheel.
The fibers of the nerve cells of the retina gather into the optic nerve (which, along with the retina itself, is actually a part of the brain, from a structural point of view; see illustration, p. 196). The optic nerve leaves the eyeball just to one side of the fovea and its point of exit is the one place in the retina where photo-receptors are completely absent. It therefore represents the blind spot. We are unaware of the existence of a blind spot ordinarily because, for one thing, the light of an object which falls on the blind spot of one eye does not fall on the blind spot
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of the other. One eye always makes it out. With one eye closed, it is easy however to show the existence of the blind spot. If one looks at a black rectangle containing a white dot and a white cross and focuses, let us say, on the dot, he will be able to locate a certain distance at which the cross disappears. Its light has fallen on the blind spot. At distances closer and farther, it reappears.
The photoreceptors when stimulated by light initiate impulses in the nearby nerve cells, and the message, conducted to the brain by the optic nerve and eventually reaching the optic area in the occipital lobe, is interpreted as light. The photoreceptors can also be stimulated by pressure, and that stimulation too is interpreted as light so that we 'see stars" as the result of a blow near the eye. Such pressure-induced flashes of light can appear if we simply close our eyelids tightly and concentrate. What we see are phosphenes (fos'feenz; "to show light" G).
The two types of photoreceptors, rods and cones, are each adapted to a special type of vision. The cones are stimulated only by rather high levels of light and are used in daylight or photopic (foh-top'ik; "light-vision" G) vision. The rods, on the other hand, can be stimulated by much lower levels of light than the cones can and are therefore involved in scotopic (skoh-top'ik; "darkness-vision" G) vision — that is, in vision in dim light.
Nocturnal animals often possess retinas containing only rods. The human eye goes to the other extreme in one respect. To be sure, the rods greatly outnumber the cones in our retinas, since the human retina contains 125 million rods and only 7 million cones. However, the macula lutea, which carries the burden of seeing, contains cones only and virtually no rods. Each cone, moreover, generally has its own optic nerve fiber, which helps maximize acuity. (Yet as many as ten or even a hundred rods may be connected to the same nerve fiber; in dim light only sensitivity is sought and acuity is sacrificed on its altar.)
Man's acuity is thus centered on photopic vision, as seems right since he is a creature of the daylight. This means, though, that
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at night acuity of vision does not exist for dim light. If one looks directly at a faint star at night, it seems to vanish altogether, because its light strikes only cones, which it is too weak to stimulate. Look to one side, nevertheless, and the star jumps into view as its light strikes rods, f Contrarily, it is because the cones become progressively less numerous away from the macula lutea that we have so little acuity in peripheral vision in daylight.}
The two forms of vision differ in another important respect, in that of color. As I shall explain shortly, specific colors involve only a portion of the range of wavelengths of light to which the eye is sensitive. The cones, reacting to high levels of light, can afford to react to this portion or that and therefore to detect color. The rods, reacting to very low levels, must detect all the light available to achieve maximum sensitivity and therefore do not distinguish colors. Scotopic vision, in other words, is in black and white, with, of course, intermediate shades of gray; a fact well expressed by the common proverb that "at night all cats are gray."
The rods contain a rose-colored visual pigment and it is that which actually undergoes the chemical change with light. It is commonly called visual purple (though it is not purple), but its more formal and more accurate name is rhodopsin (roh-dop'sin; "rose eye" G). The molecule of rhodopsin is made up of two parts: a protein, opsin, and a nonprotein portion, very similar in structure to vitamin A, which is retinene. Retinene can exist in two forms, different in molecular shape, called cis-retinene and trans-retinene. The shape of cis-retinene is such that it can combine with opsin to form rhodopsin, whereas trans-retinene cannot. In the presence of light, cis-retinene is converted to trans-retinene and, if it already makes up part of the rhodopsin molecule, it falls off, leaving the largely colorless opsin behind. (Rhodopsin may therefore be said to be bleached by light.) In the dark, trans-retinene changes into cis-retinene and joins opsin once more to form the rhodopsin.
There is thus a cycle, rhodopsin being bleached in the light and
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formed again in the dark. It is the bleaching that stimulates the nerve cell. In ordinary daylight the rhodopsin of the eyes is largely in the bleached state :iml is useless for vision. This tloes not ordinarily matter, since rhodopsin is involved in seotopic vision only and is not used in bright light. As one passes into a darkened interior, however, vision is at first almost nil because of this. It improves, as noted earlier in the chapter, by the expansion of the pupil to permit more light. It also improves because rhodopsin is gradually re-formed in the darkness and becomes available for use in dim lisiht. This period of improving vision in dim light is called dark adaptation. The bleaching of rhodopsin and the narrowiviy of the pupil on re-emergence into full light is light adaptation.
Retinene, under ideal eireumstances, is not used up in the breakdown and re-formation of rhodopsin; but the circumstances, unfortunately, are not quite ideal. Retinene is an unstable compound and, when separated from the rhodopsin molecule, has a tendency to undergo chemical change and lose its identity. Vitamin A, which is more stable, is, however, easily converted into retinene, so that the vitamin A stores of the body can be called upon to replace the constant dribbling loss of this visual pigment. The body cannot make its own vitamin A, alas, but must find it in the diet. If the diet is deficient in vitamin A, the body's stores eventually give out and retinene is not replaced as it is lost. Rhodopsin cannot then be formed, and rod vision fails. The result is that although a person may see perfectly normally in daylight, he is virtually without vision in dim light. This is night blindness, or nyctalopia (nik'tuh-loh'pee-uh; "night-blind-eye" G). Carrots are a good source of vitamin A and can help relieve this condition if added to the diet, and it is in this sense that the popular tradition that "carrots are good for the eyes" is correct.
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COLOR VISION
The wavelength of light is usually measured in Angstrom units, named for a igth-century Swedish astronomer, Anders J. Angstrom. An Angstrom unit (abbreviated A) is a very small unit of length, equal to 1/100,000,000 of a centimeter, or 1/250,000,000 of an inch. The human eye can detect light with wavelengths as short as 3800 A and as long as 7600 A. Since the wavelength just doubles at this interval, we can say that the eye can detect light over a range of one octave.
Just as there are sound waves beyond the limits of human detection, so there are light waves beyond the limits of detection, too. At wavelengths shorter than 3800 A there are, progressing down the scale, ultraviolet rays, X-rays, and gamma rays. At wavelengths above 7600 A there are, progressing up the scale, infrared rays, microwaves, and radio waves. All told, at least 60 octaves can be detected in one way or another, and of these, as aforesaid, only one octave can be detected by the eye.
We are not as deprived as this makes us seem. The type of radiation emitted by any hot body depends on its temperature, and at the temperature of the sun's surface, the major portion of the radiation is put out in the octave to which we are sensitive. In other words, throughout the eons, our eyes and the eyes of other living things have been adapted to the type of light waves actually present, in predominant measure, in our environment.
The entire range of wavelengths is commonly referred to as electromagnetic radiation because they originate in accelerating electric charges with which both electric and magnetic fields are associated.* The word "light" is usually applied to the one octave of electromagnetic radiation we can sense optically. If there is a chance of confusion, the phrase visible light can be used.
Even the one octave of visible light is not featureless, at least not to normal individuals and not in photopic vision. Just as the
" In the case of light, the accelerating electric charge is associated with the electron within the atom.
brain interprets different wavelengths of sound as possessing different pitch, so it interprets different wavelengths of light as possessing different color. Ordinary sunlight is a mixture of all the wavelengths of visible light; this mixture appears to us as white and its total absence appears to us as black. If such white light is passed through a triangular block of glass (a "prism"), refraction is not uniform. The different wavelengths are refracted by characteristic amounts, the shortest wavelengths exhibiting the highest refraction, and longer and longer wavelengths refracting progressively less and less. For this reason, the band of wavelengths are spread out in a spectrum which seems to us to be made up of the full range of colors we can see. (The spectrum reminds us irresistibly of a rainbow, because the rainbow is a natural spectrum occurring when sunlight passes through tiny water droplets left in the air after a rain has just concluded.}
The number of shades of color we see as we look along the spectrum is very large, but it is traditional to group them into six distinct colors. At 4000 A we see violet; at 4800 A, blue; at 5200 A, green; at 5700 A, yellow; at 6100 A, orange; and at 7000 A, red. At wavelengths in between, the colors exhibit various grades of intermediateness.1*
If the different wavelengths of light thus spread out into a spectrum are recombined by a second prism (placed in a position reversed with respect to the first), white light is formed again. But it is not necessary to combine all the wavelengths to do that. The igth-century scientists Thomas Young and Hermann von Hebnholtz showed that green light, blue light, and red light if combined would produce white light. Indeed, any color of the spectrum could be produced if green, blue, and red were combined in the proper proportions.
* Comparatively few animals possess a capacity for color vision, and those that do are not, apparently, quite as good at it as are the primates, including man, of course. There are interesting cases, though, where other animals may outdo us In some detail. Bees, for instance, do not respond to wavelengths in the uppermost Section of the human range. They do, however, respond to wavelengths shorter than those of violet light, wavelengths to which our eyes are insensitive. In other words, bees do not see red but do see ultraviolet.
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(Nowadays, color photography and color television make use of this. Three films, each sensitive to one of these three colors, will combine to give a photograph — or a motion picture — with a full color range; and three kinds of receiving spots on the TV screen, each sensitive to one of these three colors, will give a TV picture with a full color range.)
It seems reasonable to suppose that this is a reflection of the manner in which the human retina works. It, like the color film or the color TV screen, must have three types of photoreceptors, one sensitive to light in the red wavelengths, one to light in the blue wavelengths, and one to light in the green wavelengths. If all three are equally stimulated, the sensation is interpreted as "white" by the brain. The myriads of tints and shades the eye can differentiate are each an interpretation of the stimulation of the three photoreceptors in some particular proportion.*
Color vision is, to repeat, confined to the cones, which are not present in the far peripheral areas of the retina. They are present in increasing concentration as one approaches the macula lutea, where only cones are present. The cones themselves evidently are not identical; that is, they do not each possess all three pigments in equal proportion. Instead, there seem to be three different types of cones, each with a preponderance of its own characteristic pigment. The three types are distributed unequally over the retina. Thus, blue can be detected farther out into the retinal periphery than red can; and red, in turn, can be detected farther out than green can. At the macula lutea and in the immediately surrounding region all three are present, of course.
It sometimes happens that a person is deficient in one or more of the photoreceptors. He then suffers from color-blindness, a disorder of which there are a number of varieties and a number of gradations of each variety. One out of twelve American males shows some sort of color-vision deficiency, but very few women
* This theory does not explain all the facts in color vision, and th--e are several competing theories, some involving as many as six or seven different photoreceptors. However, the three-photoreceptor theory seems to retain most popularity among physiologists.
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are affected.* The kck, most commonly, is in the red-receptor or in the green-receptor. In either case, the person suffering the lack has difficulty in distinguishing between colors ranging from green to red. Very occasionally a person lacks all color-receptors and is completely color-blind, a condition called achromatism (ay-kroh'muh-tiz-um; "no color" G). To such a person, the world is visible only in black, white, and shades of gray.
• Color-blindness is a "sex-linked characteristic." The gene controlling it is located on the X-chromosome, of which women have two and men only one. Women have a spare, so that if one gene fails the other takes over. Men do not.
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OUR REFLEXES
RESPONSE
Any organism must be able to combine sensation with appropriate action. Some factor in the environment is sensed and some action follows. It is assumed through general experience that the action is brought about by the sensation and would not take place in its absence. If we observe someone make as though to strike us, we duck; we would not have ducked had we not experienced the sensation.
The sensation is a stimulus ("goad" L, since it goads us into the action). The action itself, which is an answer to the stimulus, is a response. This action of stimulus-and-response is characteristic of life. If we were to come across an object that did not respond to any stimulus we could think of, we would come to the conclusion that it was inanimate; or, if once alive, was now dead. On the other hand, if there was a response, we would tend to conclude instantly that the object was alive. And yet it is not a response alone that is required. If we strike a wooden plank a blow with an ax, it will respond to the stimulus of the blow by splitting; if we set a match to a mixture of hydrogen and oxygen, that will respond to the stimulus of heat by exploding. Yet this does not fool any of us into suspecting the wood or the gas mixture to be alive.
What is required of living objects is a response that maintains
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the integrity of the object; one that avoids damage or increases well-being. This is an adaptive response.
We are best acquainted with our own responses, of course. In ours there exists something we call "purpose"; we know in advance the end we are aiming at. If we are in a fight, we intend to avoid blows because we know, before the blow is received, that we shall suffer pain if we don't. What is more, we intend to strike a blow because we know in advance of the blow being struck that it will help end the fight and enforce our own desires.
Because this alliance of purpose and response is so well known to us, we tend to read purpose into the action of other creatures; even into the actions of creatures that cannot possibly have modes of thought akin to ours. For example, in observing that a green plant will turn toward the light, and knowing that light is essential to the plant's metabolism (so that receiving light contributes to its "well-being"), we are tempted to conclude that the plant turns to the light because it wants to, or because it likes the sensation, of because it is "hungry." Actually this is not at all so. The plant (as nearly as we can tell) has no awareness of its action in any sense that can be considered even remotely human. Its action is developed through the same blind and slow evolutionary forces that molded its structure.
Since light is essential to the plant's metabolism, individual seedlings (all things else being equal) which happen to possess the ability to get more than their share of sunlight will best survive. The ability may rest in a superior rate of growth enabling them to rise above the shade of neighboring plants; or, conversely, in the possession of broad leaves that grow quickly and shade the struggling neighbors, absorbing the light that would otherwise be theirs. It may be a chemical mechanism that more efficiently uses the light received, or one that enables the leaves to turn toward the light so that a "broadside blow" rather than a glancing one may be received.
Whatever the mechanism for snatching at light, the successful snatchers among plants flourish and leave more numerous de-
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scendants than their less aggressive competitors. With each generation, those responses that develop, through sheer chance, and happen to be adaptive, increasingly prevail and in the end are all but universal. If, in the course of this slow development, plant individuals arise which, through chance, tend to turn away from the light or manipulate light with lesser efficiency, such strains as they manage to establish will be quickly beaten and will drop out of the game. The same evolutionary development through chance mutation and natural selection holds for all forms of behavior, the complex varieties exhibited by man as well as the simple varieties exhibited by plants.
A nervous system is not necessary for the development of a meaningful stimulus-and-response. As I have just explained, plants, without a nervous system, will nevertheless turn portions of themselves toward the light. Such a turning in response to a stimulus is called a tropism (troh'pizm; "turning" G). Where the specific stimulus is light, the phenomenon is phototropism ("light-turning" G). The mechanism where this is accomplished is differential growth, which is in turn (see p. 90) sparked by the greater activity of auxin on the shaded side of the growing tip of the stem. When the stem receives equal stimulation from both sides, turning action ceases. (This is analogous to the manner in which we turn toward the origin of the sound, turning in the direction of that ear which gets the greater stimulus and ceasing to turn when both ears receive equal stimuli. The mechanism in ourselves is, of course, completely different from that
of plants.)
Once plant life invaded the land it was subject to the action of gravity, and geotropism ("earth-turning" G), involving an automatic and adaptive response to gravity, was developed. To take an illustration: if a seed falls into the ground "upside-down," the stem may begin its growth downward, but in the grip of negative geotropism bends about and eventually begins to grow upward, in the direction opposite the pull of gravity and, which is ultimately important, toward the light The root, contrariwise, begin-
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ning its growth upward, curves about to head downward in the direction of gravity (positive geotropism). Geotropism seems also to be mediated by auxins, but how the distribution of auxins can be affected by gravity is not yet understood. To be sure, a root will veer from its downward path if a rich source of water lies to one side. This is in response to positive hydrotropism ("water-turning" G).
Tropisms all involve slow turning through differential growth, but not all plant responses are tropisms. Some are quick responses that are almost animal-like in their resemblance to muscle action (and yet not involving muscles but, rather, such mechanisms as controlled turgor by alteration of the quantity of water present at key spots). And so there are plant species whose leaves fold by night and open by day; there are species with leaves that close at a touch; insect-digesting species with traps that close when certain sensitive trigger-projections are touched, and the like.
There are animal responses that resemble tropisms. An amoeba will move away from the light but a moth will fly toward it." Nevertheless, the responses of even very simple animals are generally more complicated than those of plants, and to call those responses tropisms would be wrong. For one thing, a tropism involves the movement of only a portion of an organism — such as the root or the stem — whereas an animal is likely to move as a whole. Such movement of a whole organism in response to a stimulus is a taxis ("arrangement" G, since the position of an organism is rearranged, so to speak, in response to the stimulus). Thus, the amoeba displays a negative phototaxis and a moth possesses a positive phototaxis.
Micro-organisms, generally, display a negative chemotaxis, which enables them to respond to a deleterious alteration in the
• We think with sardonic amusement of the moth who seems so stupidly to fly into a flame that kills it, but movement toward the light is generally adaptive behavior. The hundreds of millions of years that developed this response did so under conditions in which man-made lights did not exist and therefore posed no danger. Unfortunately for the moth, it cannot modify its response to suit the modified situation.
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chemical nature of their surroundings by swimming away, and a positive chemotaxis, which is an adaptive response to the type of chemical change brought about by the presence of something edible. There is also thigmotaxis, a response to touch, rheotaxis, a response to water currents, and a number of others.
The nature of the response may not be a simple movement toward or away. A paramecium, on encountering an obstacle, will back off a certain distance, turn through an angle of 30 degrees, and then move forward again. If it encounters the obstacle (or another obstacle) again, it repeats the process. In twelve attempts it will have made a complete turn and, by then, unless completely ringed by obstacles, it will have found its way past. But there is no true "purpose" to this, either, and however clever the little creature may seem to be in the light of our own anthropomorphic judgment, this "avoidance behavior" is a purely blind course of action developed by the forces of natural selection.
THE REFLEX ABC
The tropisms of plants and the taxis of simple animals are generalized responses of an entire organism or of a major portion of one to a very generalized stimulus. Such a generalized response to a generalized stimulus can be mediated through a nervous system, as in the case of the phototaxis of the moth, but with the development of a specialized nervous system both stimulus and response can be refined.
Special nerve-receptors can be stimulated by feebler changes in the environment than ordinary cells can be. In addition, the presence of a forest of nerve endings can make it possible to distinguish between a touch on one part of the body and a touch on another, and the two might elicit different responses. Where a nervous system is involved, in fact, a stimulus need not elicit a generalized response at all. A definite motor neuron might carry the signal required to bring about the response of a restricted portion of the body, of one set of glands, or of one set of muscles.
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Where a particular stimulus quickly and automatically produces a particular response through the action of the nervous system, we speak of a reflex ("bending back" L). The name is a good one, because the nerve impulse travels from a sense organ along a sensory nerve to the central nervous system (usually to the spinal cord but sometimes to the brain stem), and there the nerve impulse "bends back" and travels away from the central nervous system again, along a motor nerve, to bring about a response. The nerve cell connections along which the nerve impulse travels from initial sensation to final response is the reflex arc.
The simplest possible reflex arc is one consisting of two neurons, the sensory and the motor. The dendrites of the sensory nerve (•see illustrations, pp. 126 and 304) combine into a fiber that leads toward the cell body located just outside the posterior horn of the spinal cord. The axon of this nerve cell is connected by way of a synapse to the dendrites of a cell body in the anterior hom of the spinal cord. The axon of this second cell leads outward by way of an appropriate peripheral nerve to the muscle, gland, or other organ that is to give the response. Since the first neuron receives the sensation it is the receptor neuron, and since the latter effects the response it is the effector neuron. The region within the central nervous system where the two make the connection is the reflex center.
This two-neuron reflex arc is rare, but examples of it exist even in so complicated a creature as man. More common is the three-neuron reflex arc, in which the receptor neuron is connected to an effector neuron by means of an intermediate neuron called the connector neuron. The connector neuron lies wholly within the central nervous system. Even the three-neuron reflex arc is simple as far as such arcs go in highly organized creatures. In mammals the typical reflex arc is likely to have a number of connector neurons, a whole chain of them, that may lead up and down the nerve cord from one segment to others.
A complex reflex arc with numerous neurons taking part allows ample opportunity for branching. A specific receptor neuron may
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end by transmitting its nerve impulse via the various connector neurons to a number of different effectors. For instance, a painful stimulus on the hand may evoke a quick removal of the hand through the contraction of certain muscles. But for this to happen (a flexion reflex) there are opposing muscles that must simultaneously be made to relax so as not to hamper the withdrawing motion. In addition, there may be a sudden turning of the head in the direction of the painful stimulus, a sharp uncontrolled outcry, and a contortion of facial muscles. The whole variety of responses may be produced by a simple pinprick that in itself stimulates a very small number of effectors.
At the same time that a flexion reflex takes place in one limb, a crossed extensor reflex, sparked by the same stimulus, will take place in the other, stiffening and extending it. So, if we lift a leg in sudden reflex action because we have stepped on a sharp pebble on the beach, we do not usually fall over, as we might expect to since our weight is suddenly unbalanced. Instead, in equally quick reflex, the remaining leg stiffens and our weight shifts.
Another important muscular reflex is the stretch reflex. When a muscle is stretched, the proprioceptive nerve endings within it are the receptors of a reflex arc of which the effectors act to bring about a contraction that tends to counteract whatever force is bringing about the stretching. This helps keep us in balanced posture, for one thing. The balance depends upon the equal pulls of opposing muscles. If for any reason a muscle overcontracts, its opposing muscle is stretched, and that opposing muscle promptly contracts in response to the stretch, restoring the balance. If it overreacts, the first muscle is stretched and contracts in its turn.
We are not usually aware of either stimulus or response in this connection. To our conscious selves we seem only to be standing or sitting, and are completely unconcerned with the complex system of reflex arcs that must all cooperate delicately to keep us doing (in appearance) nothing, However, if we suddenly lose our balance seriously, we "catch ourselves" quite without a voluntary decision, and may indulge in violent involuntary contortions in an
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effort to regain our balance. If the stretch reflex is set off during sleep, the contracting response may again be quite sudden and violent, arousing us and giving us the impression that we "dreamed of falling."
A familiar example of the stretch reflex is the patellar reflex, or, as it is commonly called, the "knee jerk." A person being tested for the knee jerk is usuaJty asked to cross his leg and let the crossed leg hang limply. The muscle along the top surface of the thigh has its tendon attached to the bone of the lower leg. If the area just below the kneecap ("patella") is tapped lightly, that tendon is struck and the thigh muscle is momentarily stretched. This initiates a stretch reflex (this one by way of one of the rare two-neuron reflex arcs) and the muscle contracts sharply, bringing the lower leg up in a kicking motion. Because it is only a two-neuron reflex arc, it is a rapid reaction indeed.
The patellar reflex is not important in itself, but its nonap-pearance can mean some serious disorder involving the portion of the nervous system in which that reflex arc is to be found. It is so simple a reflex and so easily tested that it is a routine portion of many medical checks. Sometimes damage to a portion of the central nervous system results in the appearance of an abnormal reflex. If the sole of the foot is scratched, the normal response is a flexion reflex in which the toes are drawn together and bent downward; but if there is damage to the pyramidal tract the big toe bends upward in response to this stimulus and the little toes spread apart as they bend down. This is the Babinski reflex, named for a French neurologist, Joseph F. F. Babinski, who described it in 1896.
Just as a single receptor may in the end elicit the action of a large number of effectors, it is also possible that a particular effector or group of effectors may be brought into play by a large variety of receptors. Small individual painful stimuli over a large area of one side of the body may all cause a reflex motion of the head toward that side, and sudden pain anywhere in the body may elicit an involuntary outcry.
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The reflex arc does not involve the cerebrum, so the element of will docs not ordinarily enter. The reflex is automatic and involuntary. However, in many cases, the sensation that brings about the response is also shunted into an ascending tract and brought to the cerebrum, where it is then experienced as an ordinary sensation, usually after the reflex response is complete. If, for instance, we inadvertently touch a hot object with our hand, the hand is instantly withdrawn; withdrawn before we are consciously aware of the fact that the object was hot. The awareness follows soon enough, nevertheless, and with physical damage averted (or at least minimized) by automatic reflex action, we can then take the reasoned long-range response of moving the hot object to a safe place, or covering it, or putting a warning sign on it, or cooling it, or doing whatever seems the logical thing to do.