CHAPTER IX

THE SENSES

_Experiment 1._ =Where are the Nerves of Touch most Abundant?=--Open a pair of scissors so that the points are one eighth of an inch apart, and touch both points to the tip of the finger. Are they felt as one or as two points? Find how far they must be separated to be felt as two points when applied to the back of the neck. Record results. Caution: The person should be blindfolded, or should look away while the tests are being made. Two pins stuck in a cork will be more convenient to use than scissors.

_Experiment 2._ =Nerves of Temperature, or Thermic Nerves.=--Draw the end of a cold wire along the skin. Does the wire feel cold all the time? Repeat with a hot wire. Do you conclude that temperature is felt only in spots?

[Illustration: FIG. 119.--“COLD” SPOTS (light shading). “HOT” SPOTS (dark), skin of thigh.]

=Muscular Sense.=--_Experiment 3._ Make tests of the ability to distinguish the weight of objects weighing nearly the same, when laid by another in outstretched hand; also by laying them in the hand while it rests upon a table. Which test showed more delicate distinctions? In which were muscles brought into use? _Experiment 4._ Close the eyes and let some one move your left arm to a new position; then see if you can with the forefinger of the right hand touch the forefinger of the left hand in its new position at the first attempt.

_Experiment 5._ =Functions of the Several Parts of the Tongue.=--Test the tip, edges, and back of the tongue with sugar, vinegar, quinine, and salt. Where is the taste of each most acute? Record results.

=Flavors.=--_Experiment 6._ Blindfold a member of the class, and while he holds his nostrils firmly closed by pinching them, have him place successively upon his tongue a bit of potato and of onion. Can he distinguish them? _Experiment 7._ Mark _F_ after each of the following foods that have a flavor (see text): vanilla, apple, lemon, beef, peaches, grapes, coffee, onion, potato, cinnamon.

_Experiment 8._ =A Smelling Contest.=--Place the following and other things having taste in vials around which paper has been pasted to conceal their contents: pepper sauce, vinegar, kerosene, flavoring extracts (diluted), several perfumes, iodine, bits of banana, lemon, apple, etc. Number the vials and have pupils test and write results in a list. Correct the lists and announce pupil having keenest sense of smell.

_Experiment 9._ =A tasting contest= may be arranged in a similar way. Smelling and tasting tests should be made quickly as these senses are soon dulled by repeating a sensation.

_Experiment 10._ =Advantage of Two Eyes over One.=--Try to touch forefinger to something held by another at arm’s length from you, bringing the finger in from the side: (1) with one eye closed; (2) with both eyes open. Result? Conclusion? We tell the distance of an object by the amount of convergence of the eyeballs needed to look at it.

_Experiment 11._ =Duration of Impression.=--Whirl a stick with a glowing coal on one end (see Fig. 123).

_Experiment 12._ =Color Blindness.=--Provide a number of yarns of different tints, and the same tints. Test color blindness by having each pupil match tints and assort the yarns.

_Experiment 13._ =Fatigue of Optic Nerve.=--Gaze long and steadily at a moderately bright object, then close the eyes. Result? Conclusion?

_Experiment 14._ =Dissection of Eye.=--The eye of an ox is an interesting subject for dissection. The lens is like a clear crystal. Make out all parts named in the text (see Fig. 122).

_Experiment 15._ =Image formed by a Convex Lens.=--For a few cents obtain from a jeweler a convex lens, or use a strong pair of spectacles worn by an old person. Hold the lens a few feet from a window (darken any other windows near). A little beyond the lens hold a white card or book open at a blank page to catch the image. Have some one walk before the window.

_Experiment 16._ =Work of Iris.=--Notice the size of the pupils. Cover one eye with the hand for a few minutes. Uncover and look in a mirror. Gaze at bright window and look again in the mirror. Conclusion? Do the two pupils still keep the same size when one eye is shaded?

_Experiment 17._ =Accommodation.=--By holding your finger or a pencil in line with writing on the blackboard, you find that you cannot see both finger and blackboard distinctly at the same time--first one and then the other is distinct. Explain (see text).

_Experiment 18._ =Astigmatism= (effect of unequal curvature of cornea or lens along certain lines). With end of crayon draw about twelve straight, even lines crossing at one point on the blackboard. Have the lines of equal distinctness. How many pupils report that the lines in certain directions are blurred? Inquire whether these pupils have frequent headaches from eye strain.

_Experiment 19._ =Can Sound reach the Ear through the Bones?=--Hold a watch between the lips and notice its ticking. Close the teeth down upon it and notice any change in the sound. Cover one, then both ears, and note the result.

_Experiment 20._ =Test keenness of hearing= by having pupils walk away from a ticking watch until it becomes inaudible. Test each ear. A “stop” watch is preferable.

_Experiment 21._ =Advantage of Two Ears over One.=--Have the class stand in a circle. Blindfold some one and place him in the middle of the circle. Let various pupils clap the hands as the teacher points to each. Can the blindfolded one point in the direction whence the sound comes? Stop one ear with a handkerchief and repeat. Result? Conclusion? From what two points in the circle does the sound fall upon both ears alike?

_Experiment 22._ =The Cause of Nasal Tones.=--Let a pupil go to the back of the room and read a paragraph, and hold his nose until partly through the reading. Or the teacher may read with his face and hand hidden by a large book. Let the other pupils raise their hands when they notice a change in the quality of the reader’s voice. Does the experiment show that a “nasal” tone comes partly through the nose or through the mouth only? Does stoppage of the nostrils by catarrh cause a nasal tone?

=Five Differences between Special and General Sensation.=--First, the nerves of special sense all end in special organs at the surface; for instance, the touch corpuscles are for touch, the eye is for sight, etc. There are many nerves in the body that do _not end in special organs_; these nerves give what is called general sensation. A second difference is that general sensation _tells of the condition of the interior of the body_, while special sensations tell us of the condition of the surface of the body and of the outside world. Third, _general sensations are not so exact_ as the reports of the special senses. One can locate a point on the skin that has been touched much more accurately than he can locate an internal pain. A fourth difference is that the meaning of each special sensation must be learned (usually in infancy); but the _meaning of general sensations is inherited_. This inherited knowledge of what general sensations mean is also called instinct. Fifth, the _sympathetic nerves usually bring general sensations_; the spinal and cranial nerves usually bring special sensations.

=Examples of general sensations= are hunger, thirst, satiety, nausea, faintness, giddiness, fatigue, weight, aching, shuddering, restlessness, blues, creepy feeling, tingling, sleepiness, pain, illness. Any nerve can convey the general sensation of pain, if injured along its course. If a nerve of touch is cut, there is no sensation of touch, but of pain. Touch sensations come only from the ends of the nerves. General sensations are of many kinds. We are only half conscious of some of them; many of them are hard even to describe.

=Hygiene of the General Sensations.=--General sensation is an invaluable aid to the health. Without it as a guide, the body could not remain alive a single day. _Pain_ should be heeded as our best friend, and not killed with poisonous drugs as if it were our worst enemy. We should not deaden the stomach ache with an after-dinner cigar. If we do not go to bed when sleepy, the _desire for sleep_ may leave us, and we will undergo untold suffering from sleeplessness. _Thirst_ should be satisfied with cool water, which quenches it the best; he who makes his teeth ache with ice water will inflame his stomach and be continually thirsty. He who does not stop eating when his _hunger_ is satisfied, will distend his stomach with food, and the stretched organ will be harder to satisfy thereafter; in fact, eating after a feeling of satiety may cause indigestion so that the cells will not get the food. A dyspeptic is always hungry, for the cells are starving. _Fatigue_ of body or mind gives us wise counsel; but this feeling may be deadened by alcohol or tobacco, and work continued until the body is injured. We should heed the warning of pain or fatigue or restlessness as promptly as an engineer heeds a red flag on the railway track. One who uses narcotics acts like a reckless engineer who removes the danger signal and goes ahead, hoping by good luck to escape an accident.

Most of the =nerves of touch= end in papillæ of the dermis as _microscopic, egg-shaped bodies_ (Fig. 120). There are also many in the interior of the mouth, especially on the tongue. On the palms they are arranged in curved lines, and on the tips of the fingers they are in circular lines, with one papilla in the center. The delicacy of the sense of touch varies very much in different parts of the skin. _This delicacy refers to two things_: the ability to feel the slightest pressure and the ability to tell the exact point of the skin that is touched. A lighter pressure can be felt on the forehead and temples than with any part of the body. (Why is it best for this to be the case?) The greatest delicacy in locating the point of the skin touched is found to be located in the tip of the tongue, the lips, and the ends of the fingers (Exp. 1). (Why is it best that this is so?) This delicacy is least in the middle of the back. The delicacy varies with the number of touch corpuscles in different parts of the skin. The sense of touch is capable of great cultivation, as in the case of the blind.

[Illustration: FIG. 120.--DIFFERENT KINDS OF TOUCH BODIES AT ENDS OF NERVES.

_A_, from cornea of the eye; _B_, from the tongue of a duck; _C_, _D_, _E_, from the skin of the fingers. (Jegi.)]

=The temperature sense= is given by special nerves called the thermic nerves (Exp. 2). That the _thermic nerves are easily fatigued_ is noticed soon after entering a bath of hot water; it is also shown by the fact that in cold countries the nose or ears of a person may freeze without his feeling it.

=The Muscular Sense.=--_The special sense of touch gives some sense of weight._ A weight upon the skin must be increased by one third before it feels heavier, but by lifting an object so as to _bring into action the muscular sense residing in nerves ending in the muscles_ an increase of only one seventeenth of the original weight can be noticed (Exp. 3). This sense gives us a continual _account of the position of the limbs_ (Exp. 4).

=The end organs of taste= are located in the papillæ of the tongue. The tongue has a fuzzy look because of the numerous papillæ.

=The principal tastes= are only four; namely, sweet (tasted chiefly by tip of tongue), sour and saline (sides of tongue), bitter (tasted on the back of tongue) (Exp. 5).

=The nerves of smell= end in the mucous membrane of the upper half of the two nasal chambers; the _fibers are spread over the upper proportion of the walls_. The direct current of air does not pass as high as these nerve endings; hence sniffing aids the perception of odors. This sense is able to bring up the associations of early life more powerfully than any of the senses. The odor of a flower like one that grew in an old garden can almost restore the consciousness of the past. _We smell gases only_; solids and liquids cannot affect this pair of nerves (Exp. 8).

=Flavors.=--The tastes that we call flavors are really smells. We confuse them with taste, because they accompany food that is in the mouth. Name some foods that seem “tasteless” when one has a severe cold in the head. Why is this? Some of the most repulsive drugs can be easily swallowed if the nose is held (Exp. 6 and 7).

=Hygiene of the Senses of Taste and Smell.=--A savage or a beast uses the senses of taste and smell to find out whether things are good to eat or not. If a civilized man’s senses are not perverted, and he eats only simple foods that have a pleasant taste, they will not injure him or cause him sickness. Things that are poisonous usually have unpleasant tastes and often have unpleasant odors. These senses are naturally of wonderful delicacy. They can be cultivated to a still more remarkable degree, or they can be blunted and almost destroyed. Chronic catarrh dulls or destroys the sense of smell. The loss or even the weakening of the perception of flavors is an injury to the working of the closely related sense of taste. When a person loses the enjoyment of delicate flavors, he wants food to have strong seasoning and more decided taste to prevent it from being insipid. Everything must be either very greasy or very sweet or very salty or very sour, to please his degenerate senses. Wheat, corn, and other grains have each its own pleasant taste, yet such persons must have lard in their bread because they are not capable of appreciating anything with a delicate taste. In England, butter is not salted and its delicate taste is enjoyed; in America, salt is added to preserve it, and most people have come to prefer the strong taste of salty butter to the delicate taste of pure butter, and do not like it unless its true taste is partly hidden by the taste of salt (Exp. 9).

=Deceiving the Sense of Taste.=--The habit of using narcotics like _tea_ and _coffee_ is usually begun by concealing the repulsive bitter taste of the substance by mixing sugar, cream, and other agreeable things with it. Licorice is sometimes mixed with _tobacco_ to weaken its biting taste. _Pure alcohol_ would never be drunk by any one who had the least respect for the sense of taste, but the agreeable flavor of grapes, apples, and other fruit which still remains in wine, cider, and brandy, conceals the repulsive taste of the alcohol. _Beer_ has the insipid taste of grain which has undergone decomposition or partial rotting, and hops are added because the strong bitter taste of hops is needed to hide the stale, rancid taste of the rotted grain. _Eggnog_ is made of eggs, a nourishing food; sugar, which has an agreeable taste; water, a refreshing drink, and alcohol, a fiery poison. A very good eggnog is often made without alcohol, but a good one could hardly be made with any of the pleasant ingredients left out. The best eggnog is made by using the fresh juice of lemon, orange, or grape, instead of alcohol.

=Effect of Narcotics.=--Tobacco, alcohol, opium, and other narcotics dull the senses of taste and smell and prevent the enjoyment of delicate flavors. They accomplish this as much by their effect upon the brain as upon the nerves themselves.

=It is Wrong to eat Food that is not Relished.=--Unpalatable food is not likely to be well digested. It is a law of the body that _the food which is enjoyed the most is digested the best_. This applies to a hungry person eating food with its own honest taste, not to food disguised by the taste of something else. The rule does not apply to a taste perverted by having been forced to become accustomed to poisonous things. People who munch their food slowly enjoy the pleasures of taste the most, and digest their food the best. The nerves of taste and smell easily become fatigued. The first whiff from a cologne bottle is the strongest. Highly flavored foods should be eaten moderately, if we would obtain the greatest enjoyment from them.

THOUGHT QUESTIONS.--=1. Interfering with the Body.= What is the natural direction of growth of the big toe? =2.= Think of six evil results, direct or indirect, which will follow from displacing it by tight shoes (p. 48). =3.= Which part of the spinal column, designed in infinite wisdom to be most flexible, do some people try to make the most inflexible? =4.= The mobility of the false and floating ribs was intended as a blessing. Some people interpret the blessing as an opportunity to do what? =5.= Name six articles which warn us to avoid them by their bitter, burning, or nauseating tastes, yet which are used by man. =6.= Name six feelings which are intended as warnings for our guidance, but which are commonly disregarded.

=The eyes= on the rays of the starfish are mere spots of pigment. Insects have lenses in their eyes. The eyes of vertebrates are all formed on the same general plan as the human eye.

=The eyeballs= are globes about an inch in diameter. They are placed in deep, bony sockets, called _orbits_, in the front part of the skull. The optic nerve, other nerves, and several large blood vessels pass to the eye through a hole in the back of the orbit. A soft cushion of fat is in the orbit behind the eyeball. A pressure upon the eyeball causes the eye to sink into the socket, for the fat yields to the pressure. This is a protection to the eye.

=The eyelids= protect the eyes from dust, and at times from the light. They are aided in this by the eyelashes.

[Illustration: FIG. 121.--TEAR GLANDS AND DUCTS of right eye. (Jegi.)]

=The tears= are formed by _tear glands_ situated above the eyeball in the portion of the orbit farthest from the nose, just beneath the bony brow where it feels the sharpest (Fig. 121). They are about the size of almonds. A saltish liquid is continually oozing from the tear glands and passing over the eyeball; it is carried into the nose through the _nasal duct_ (Fig. 121). The tears reach this duct through _two small canals_, which open into the eye in the little fleshy elevation at the inner corners of the eye (Fig. 121). The opening of one of the canals may be seen by looking into a mirror. Sometimes these canals are stopped up, and what is called a “weeping eye” results. A temporary stoppage may occur during a cold in the head.

Tears prevent friction between eye and lid. Winking applies the tears to the ball. Small glands along the edges of the lids form a kind of oil which usually prevents the tears from flowing over the lids. Sometimes this oily secretion is so abundant, especially during sleep, as to cause the lids to stick together. The mucous membrane of the eyelids continues as a transparent membrane (the conjunctiva) which passes over the front of the ball.

[Illustration: FIG. 122.--THE ANATOMY OF THE EYE.]

=The globe of the eye= consists of its outer wall and the soft contents (Fig. 122). The wall has three layers or coats. The outer coat is the tough _sclerotic_ (Greek, _skleros_, hard), composed of dense connective tissue (Exp. 14). It gives strength and firmness to the eyeball. It shows between the lids as the “white of the eye.” It is white and opaque except in front; there it bulges out to form the transparent _cornea_. This clear portion of the wall may be seen by looking at the eye of another from the side.

The second coat, called the _choroid_, consists of blood vessels and a loose connective tissue containing many dark brown or black pigment granules. The choroid absorbs superfluous light. Cats’ eyes shine at night because this coat in their eyes reflects some light. The choroid separates from the sclerotic toward the front of the eye and forms the colored _iris_. The iris makes the eyes beautiful, and it also serves the useful purpose of regulating the amount of light. The hole in the iris is called the _pupil_ (Exp. 15).

The third and innermost coat, the sensitive pinkish layer called the _ret′in-a_, is the most important and characteristic tissue in the eye. It receives the light rays, and retains the image for a fraction of a second (Exp. 11). Hence the pictures in a kinetoscope (Fig. 123) appear as one moving picture. The retina is made chiefly of the fibers of the optic nerve. This nerve contains about five hundred thousand fibers, and enters at the back of the ball. The spot where it enters contains no nerve endings and is not sensitive to light. It is called the _blind spot_. The spot where the light most often falls is most sensitive to light. It is the _yellow spot_ (Fig. 122).

[Illustration: FIG. 123.--STROBOSCOPE, the original of the kinetoscope. The observer looks through the slits of a rapidly revolving disk and a new image falls on the retina before the last image has faded. Compare the pictures in the figure.]

=Test for the Blind Spot.=--In this experiment shut the right eye and be careful not to let the left eye waver.

* Read this line slowly. Can you see the star all the time? (If so, hold the book farther or closer and repeat.)

=Within the coats of the ball=, like the pulp within the rind of an orange, are the soft contents, divided into three parts. The first is a watery liquid in front, which serves to keep the cornea bulged out (Fig. 122). It is called the _a′que-ous humor_. The main cavity of the ball is occupied by a clear, jellylike substance called the _vit′re-ous humor_, which serves to keep the ball distended. Back of the iris, and separating the two humors just named, is the _crys′tal-line lens_, a beautiful clear lens, convex or rounded out on both sides (Exp. 14). It serves to bring the light to a focus on the retina, thereby forming images of outside objects.

=The eye, like a camera=, has a dark lining, the choroid; the retina corresponds to the sensitive plate, and the lens brings the rays to a focus on it and forms the image.

[Illustration: FIG. 124.--CROSSING OF OPTIC NERVES showing that one nerve reaches same half of both eyes.]

=The Path of Light in the Eye.=--The light enters through the transparent cornea and passes through the aqueous humor. As it goes through the pupil, the iris shuts off all the light that is not needed. The crystalline lens receives the light that has been allowed to pass, and so bends the rays that by the time they have passed through the vitreous humor they fall upon the retina in just the right way to form a tiny image of anything outside (Exp. 11). The choroid absorbs any light that passes the retina. The iris and choroid of albinos have no pigment; hence albinos squint their eyes to shut out some of the light.

=Accommodation.=--In order to focus the light upon the retina, the _lens must change shape for every change in the distance of the object looked at_ (see Fig. 125). The shape of the lens can be readily changed, for it is elastic and has muscular fibers around its edges (Exp. 17).

[Illustration: FIG. 125.--Change of lens in accommodation. (Jegi.)]

=Defects in the Eye.=--Some eyeballs are too long, and the lens brings the rays to a focus before they reach the retina. Such eyes are _nearsighted_ (Fig. 126) and require glasses that round inward (concave). Some eyeballs are too flat, and the rays are not brought to a focus soon enough. Such eyes are _farsighted_ and require glasses that round outward (convex). See Fig. 127. (Repeat Exp. 15.)

[Illustration: FIG. 126.--(1) NEARSIGHTED EYE (ball too long), which only focuses rays for near objects (2) when concave glasses are used (3).]

[Illustration: FIG. 127.--FARSIGHTED EYE (ball too short) which needs convex lens to focus rays upon retina.]

=Care of the Eyes.=--Because the eyes can do a large amount of work without giving pain, they are often abused. When reading or doing intricate work, turn the eyes from the work occasionally and look at some distant object; stop work before the eyes are tired. Twilight of early evening has ruined many good eyes. You should always stop work before the twilight begins, for the light fades so gradually that you will surely be straining the eyes before you know it. Do not work with the light in front; the glare of the light makes objects appear dim. The light should come from above, and (for right-handed people) from the left. Do not read papers or books printed in fine type. We should not read when convalescing from illness; with the head bent down; when the eyes are sore; in jolting cars. Heating the eyes by a burner, or drying the eyeballs in a dry, stove-heated atmosphere, using a light without a shade, cause trouble with students’ eyes. Of what are blood-shot eyes often a sign? Our eyes are best suited for seeing at a distance because primitive man had no houses, books, sewed clothes. Effort is required to shape the lens for seeing near objects. Most cases of nearsightedness begin when children are taught to read under eight years old. The eyes are sometimes injured by the use of tobacco.

THOUGHT QUESTIONS. =The Eye.=--=1.= The eye is shielded from blows by bony projections of ____, ____, and ____. =2.= The hairs of the eyebrows lie inclined toward ____, in order to turn ____ from the ____. =3.= I find by trying it that I (can or cannot?) see the position of a window with my eyes closed. =4.= The pupil appears to be black, because no ____ is ____ from the interior wall of the eye. I know that the iris is partly muscle, because it ____ the size of the ____.

=Sound.=--Anything that is sending off sound does so by vibrating, or shaking to and fro, very rapidly. For instance, a vibrating violin string sets every particle of air near it swinging to and fro. The nearest particles of air strike the next ones and bounce back, these in turn strike against others, and thus vibrations called sound waves are sent through space in all directions from the sounding body. We feel these waves with the ear.

=The ear= consists of three portions: the _external_ ear, the _middle_ ear (or drum), and the _internal_ ear (or labyrinth, see Fig. 128). The cranial nerve connecting the ear with the brain is called the _auditory_ nerve. The outer and middle ear pass on the vibrations of air to the ends of the fibers of the auditory nerve in the internal ear.

[Illustration: FIG. 128.--MIDDLE AND INTERNAL EAR (greatly enlarged).]

=The external ear= consists of a large wrinkled _cartilage_ on the exterior of the head and a _canal_ leading from it, called the _meatus_. This passage is closed at its inner end by the drum membrane or _drum skin_. It is often called the drum, but this name is properly applied to the whole middle ear. A trial will show that the drum skin cannot be seen even with the aid of a bright light, for the passage is slightly curved (see Fig. 128). Hence a missile or a flying insect cannot go straight against the ear drum. The skin lining this passage contains _wax glands_, which secrete a bitter sticky wax, which helps to keep the passage flexible. This wax catches dust and usually stops insects that may enter. If an insect enters the ear, it may often be coaxed out by a bright light held close to the ear. The ear wax in a healthy ear dries with dust and scales of epidermis and falls out in flakes, thus cleansing the ear. It is unwise to probe into the ear with a hard object or even with the corner of a towel. It is not necessary to insert the finger in the meatus to cleanse it; it is one inch long, but only about one fourth inch across. (How large is the little finger?) The cartilaginous ears on the sides of the head should be carefully washed because of their many crevices. If ear wax is deposited too fast, it will cause temporary deafness and earache. It may be syringed out with warm water. Earache is usually caused by a small boil which requires time to relieve itself by bursting. Warm water poured into the upturned ear, or hot flannels or compresses applied to the side of the head will lessen the suffering. Each ear has three muscles for moving it. Once they were doubtless useful to all, but like the scalp muscle they have become so weakened by disuse as to be useless to most people. They are vestigial organs.

=The middle ear=, or drum chamber, contains air (Fig. 128). It is separated from the outer ear by the drum membrane. It contains three bones which stretch across it and conduct the sound waves from the drum membrane to the inner ear. State the order in which they are placed (see Fig. 128). The middle ear is connected with the pharynx by a tube (the _Eustachian tube_; pronounced yoo-stake´e-an, see Fig. 128). This tube is opened every time we swallow. It allows the air from the throat to enter the middle ear and keep the air pressure equal on each side of the drum skin. This tube and the middle ear are lined with mucous membrane.

A _cold in the head_ or a sore throat may extend through this tube to the middle ear and affect the hearing. This occurs because the tube is closed by congestion of its lining; the air of the middle ear may be partly absorbed, and the pressure of the outside air may cause the drum membrane to bulge inward, and to be stretched so tight that it cannot vibrate freely.

=The inner ear= is called the _labyrinth_, because of its winding passages. There is a spiral passage called the _snail shell_ and three simpler passages called the _loops_ (Fig. 128). The inner ear is filled with a limpid liquid which conveys the vibrations to the _ends of the auditory nerve_ found in the snail shell. If the auditory nerve or labyrinth becomes diseased, the deafness is probably incurable. Quinine and other drugs may cause deafness.

=Sense of Equilibrium.=--Some fibers of the auditory nerve end in the loops and are not believed to be used in hearing. It is believed that each loop acts like a carpenter’s level, and the varying pressure of the fluid upon the nerves in the loops tells us the position of the body and constitutes the sense of equilibrium. There are how many of these loops in each ear? (Fig. 128.)