[Ill.u.s.tration: Fig. 30.--Two views of the internal ear. These views show the shape of the internal ear cavity. The sense organs lie inside this cavity. Notice how the three semi-circular ca.n.a.ls lie in three perpendicular planes. (Figure text: cochlea, vestibule, 3 Ca.n.a.ls)]

The ear is about as complex a piece of mechanism as the eye. We speak of the "outer", "middle" and "inner" ear. The outer, in such an animal as the horse, serves as a movable ear trumpet, catching the sound waves and concentrating them upon the ear drum, or middle ear. The human external ear seems to accomplish little; it can be cut off without noticeably affecting hearing. The most essential part of the external ear is the "meatus" or hole that allows the sound waves to pa.s.s through the skin to the tympanic membrane or drum head. The sound waves throw this membrane into vibration, and the vibration is transmitted, by an a.s.sembly of three little bones, across the air-filled cavity {196} of the middle ear to an opening leading to the water-filled cavity of the inner ear. This opening from the middle to the inner ear is closed by a membrane in which one end of the a.s.sembly of little bones is imbedded, as the other end is imbedded in the tympanic membrane; and thus the vibrations are transmitted from the tympanic membrane to the liquid of the inner ear. Once started in this liquid, the vibrations are propagated through it to the sense cells of the cochlea and stimulate them in the way already suggested.

[Ill.u.s.tration: Fig. 31.--A small sample of the sense cells of the cochlea. The hairs of the sense cells are shaken by the vibration of the water, and pa.s.s the impulse back to the end-brushes of the auditory axons, The tectorial membrane looks as if it might act as a damper, but may be concerned, as "accessory apparatus," in the stimulation of the hair cells. The basilar membrane consists in part of fibers extending across between the ledges of bone; these fibers are arranged somewhat after the manner of piano strings, and have suggested the "piano theory" of hearing, to be mentioned later in the chapter. (Figure text: water s.p.a.ce, membrane, Tectorial membrane, bone, soft tissue, basilar membrane, auditory axons to brain stem, nerve cells of auditory nerves, auditory hair cells with end brushes of auditory axons)]

Further study of the accessory apparatus of the eye and ear can be recommended as very interesting, but the little that has been said will serve as an introduction to the study of sensation.

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a.n.a.lysis of Sensations

Prominent among the psychological problems regarding sensation is that of a.n.a.lysis. Probably each sense gives comparatively few elementary sensations, and many blends or compounds of these elements. To identify the elements is by no means a simple task, for under ordinary circ.u.mstances what we get is a compound, and it is only by carefully controlling the stimulus that we are able to get the elements before us; and even then the question whether these are really elementary sensations can scarcely be settled by direct observation.

Along with the search for elementary sensations goes identification of the stimuli that arouse them, and also a study of the sensations aroused by any combination of stimuli. Our task now will be to ask these questions regarding each of the senses.

The Skin Senses

Rough and smooth, hard and soft, moist and dry, hot and cold, itching, tickling, p.r.i.c.king, stinging, aching are skin sensations; but some of these are almost certainly compounds. The most successful way of isolating the elements out of these compounds is to explore the skin, point by point, with weak stimuli of different kinds. If a blunt metal point, or the point of a lead pencil, a few degrees cooler than the skin, is pa.s.sed slowly over the skin, at most points no sensation except that of contact arises, but at certain points there is a clear sensation of cold. Within an area an inch square on the back of the hand, several of these _cold spots_ can be found; and when the exploration is carefully made, and the cold spots marked, they will be found to give the same sensation every time. Subst.i.tute a metal point a few {198} degrees warmer than the skin, and a few spots will be found that give the sensation of warmth, these being the _warmth spots_. Use a sharp point, like that of a needle or of a sharp bristle, pressing it moderately against the skin, and you get at most points simply the sensation of contact, but at quite a number of points a small, sharp pain sensation arises. These are the _pain spots_. Finally, if the skin is explored with a hair of proper length and thickness, no sensation at all will be felt at most points, because the hair bends so readily when one end of it is pressed against the skin as not to exert sufficient force to arouse a sensation; but a number of points are found where a definite sensation of touch or contact is felt; these are the _touch spots_.

No other varieties of "spots" are found, and the four sensations of touch, warmth, cold and pain are believed to be the only elementary skin sensations. Itch, stinging and aching seem to be the same as pain. Tickle is touch, usually light touch or a succession of light touches. Smooth and rough are successions of touch sensations. Moist is usually a compound of smooth and cold. Hard and soft combine touch and the muscular sensation of resistance.

Hot and cold require more discussion. The elementary sensations are warmth and coolness, rather than hot and cold. Hot and cold are painful, and the fact is that strong temperature stimuli arouse the pain spots as well as the warmth or cold spots. Hot, accordingly, is a sensation compounded of warmth and pain, and cold a sensation composed of coolness and pain. More than this, when a cold spot is touched with a point heated well above the skin temperature (best to a little over 100 Fahrenheit), the curious fact is noted that the cold spot responds with its normal sensation of cold. This is called the "paradoxical cold sensation". From this fact it is probable that a hot object excites the cold sensation, along with those of warmth and {199} pain; so that the sensation of heat is a blend of the three. Another curious fact is that a very cold object produces a burning sensation indistinguishable from that of a hot object; so that the sensation of great cold, like that of heat, is probably a blend of the three elementary sensations of warmth, cold and pain.

[Ill.u.s.tration: Fig. 32.--Diagram of various sorts of sensory end-organ found in the skin.

A is a hair end-organ; the sensory axons can be seen coiling around the root of the hair; evidently a touch on the hair, outside, would squeeze the coiled axon and stimulate it. The hair is a bit of "accessory apparatus."

B is a touch corpuscle, consisting of a coiled axon-end surrounded by a little cone of other tissue.

C is an end-bulb, presumably belonging to the temperature sense. It has, again, a coiled axon-end surrounded by other tissue. The "coils" are really much more finely branched than the diagram shows.

D is a free-branched nerve end, consisting simply of a branched axon, with no accessory apparatus. It is the pain-sense organ.

E is a corpuscle of a type found in the subcutaneous tissue, as well as in more interior parts of the body. It contains an axon-end surrounded by a layered capsule.]

The stimulus that arouses the touch sensation is a bending of the skin. That which arouses warmth or cold is of {200} course a temperature stimulus, but, strange as it may seem, the exact nature of the effective stimulus has not been agreed upon. Either it is a warming or cooling of the skin, or it is the existence of a higher or lower temperature in the skin than that to which the skin is at the moment "adapted". This matter will become clearer when we later discuss adaptation. The stimulus that arouses the pain sensation may be mechanical (as a needle p.r.i.c.k), or thermal (heat or cold), or chemical (as the drop of acid), or electrical; but in any case it must be strong enough to injure or nearly to injure the skin. In other words, the pain sense organ is not highly sensitive, but requires a fairly strong stimulus; and thus it is fitted to give warning of stimuli that threaten injury.

Several kinds of sensory end-organ are found in the skin. There is the "spherical end-bulb", into which a sensory axon penetrates; it is believed to be the sense organ for cold. There is the rather similar "cylindrical end-bulb" believed to be the sense organ for warmth.

There is the "touch corpuscle", found in the skin of the palms and soles, and consisting, like the end-bulbs, of a ma.s.s of accessory cells with a sensory axon ramifying inside it; this is an end-organ for the sense of touch. There is the hair end-organ, consisting of a sensory axon coiled about the root of the hair; this, also, is a touch receptor. Finally, there is the "free-branched nerve end", consisting simply of the branching of a sensory axon, with no accessory apparatus whatever; and this is the pain receptor. Perhaps the pain receptor requires no accessory apparatus because it does not need to be extremely sensitive.

Now since we find, in the skin, "spots" responsive to four quite different stimuli, giving four quite different sensations, and apparently provided with different types of end-organs, it has become customary to speak of four skin senses in place of the traditional "sense of touch". We {201} speak of the pain sense, the warmth sense, the cold sense, and the pressure sense, which last is the sense of touch proper.

The Sense of Taste

a.n.a.lysis has been as successful in the sense of taste as in cutaneous sensation. Ordinarily we speak of an unlimited number of tastes, every article of food having its own characteristic taste. Now the interior of the mouth possesses the four skin senses in addition to taste, and many tastes are in part composed of touch, warmth, cold or pain. A "biting taste" is a compound of pain with taste proper, and a "smooth taste" is partly touch. The consistency of the food, soft, tough, brittle, gummy, also contributes, by way of the muscle sense, to the total "taste". But in addition to all these sensations from the mouth, the flavor of the food consists largely of odor. Food in the mouth stimulates the sense of smell along with that of taste, the odor of the food reaching the olfactory organ by way of the throat and the rear pa.s.sage to the nose. If the nose is held tightly so as to prevent all circulation of air through it, most of the "tastes" of foods vanish; coffee and quinine then taste alike, the only _taste_ of each being bitter, and apple juice cannot be distinguished from onion juice.

But when the nose is excluded, and when cutaneous and muscular sensations are deducted, there still remain a few genuine tastes.

These are sweet, sour, bitter and salty--and apparently no more. These four are the elementary taste sensations, all others being compounds.

The papillae of the tongue, with their little "pits" already spoken of, correspond to the "spots" of the skin, with this difference, however, that the papillae do not each give a single sensation. Some of them give only two, some only three of the four tastes; and the bitter taste is aroused princ.i.p.ally from {202} the back of the tongue, the sweet from the tip, the sour from the sides, the salty from both tip and sides.

The stimulus to the sense of taste is something of a chemical nature.

The tasteable substances must be in solution in order to penetrate the pits and get to the sensitive tips of the taste cells. If the upper surface of the tongue is first dried, a dry lump of sugar or salt laid on it gives no sensation of taste until a little saliva has acc.u.mulated and dissolved some of the substance.

Exactly what is the chemical agent that produces a given taste sensation is a problem of some difficulty. Many different substances give the sensation of bitter, and the question is, what there is common to all these substances. The sweet taste is aroused not only by sugar, but by glycerine, saccharine, and even "sugar of lead" (lead acetate). The sour taste is aroused by most acids, but not by all, and also by some substances that are not chemically acids. Thus the chemistry of taste stimuli involves something not as yet understood.

Though there is this uncertainty regarding the stimulus, on the whole the sense of taste affords a fine example of success achieved by experimental methods in the a.n.a.lysis of complex sensations. At the same time it affords a fine example of the fusion of different sensations into characteristic _blends_. The numerous "tastes" of every-day life, though found on a.n.a.lysis to be compounded of taste, smell, touch, pain, temperature and muscle sensations, have the effect of units. The taste of lemonade, for example, compounded of sweet, sour, cold and lemon odor, has the effect of a single characteristic sensation. It can be a.n.a.lyzed, but it ordinarily appears as a unit.

This is true generally of blends; indeed, what we mean by blending is that, while the component sensations are still present and can be found by careful attention, they are not simply present together {203} but are compounded into a characteristic total. Each elementary sensation entering into the blend gives up some of its own quality, as, in the case of lemonade, neither the sweet nor the sour is quite so distinct and obtrusive as either would be if present alone. The same is true of the lemon odor, and it is true generally of the odor components that enter into the "tastes" of food. Were the odor components in these tastes as clear and distinct as they are when the same substance is smelled outside the mouth, we could not fail to notice that the "tastes" were largely composed of odor. The obtrusive thing about a blend is the total effect, not the elementary sensations that are blended.

The Sense of Smell

The great variety of odors long resisted every attempt at psychological a.n.a.lysis, largely because the olfactory end-organ is so secluded in position. You cannot apply stimuli to separate parts of it, as you can to the skin or tongue. But, recently, good progress has been made, [Footnote: By Henning.] by a.s.sembling almost all possible odors, and becoming thoroughly acquainted with them, not as substances, but simply as odors, and noting their likenesses and differences. It seems possible now to state that there are _six elementary odors_, as follows:

1. Spicy, found in pepper, cloves, nutmeg, etc.

2. Flowery, found in heliotrope, etc.

3. Fruity, found in apple, orange oil, vinegar, etc.

4. Resinous, found in turpentine, pine needles, etc.

5. Foul, found in hydrogen sulphide, etc.

6. Scorched, found in tarry substances.

These being the elements, there are many compound odors. The odor of roasted coffee is a compound of resinous and scorched, peppermint a compound of fruity and spicy.

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Each elementary odor corresponds to a certain characteristic in the chemical const.i.tution of the stimulus.

The sense of smell is extremely delicate, responding to very minute quant.i.ties of certain substances diffused in the air. It is extremely useful in warning us against bad air and bad food. It has also considerable esthetic value.

Organic Sensation

The term "organic sensation" is used to cover a variety of sensations from the internal organs, such as hunger, thirst, nausea, suffocation and less definite bodily sensations that color the emotional tone of any moment, contributing to "euphoria" and also to disagreeable states of mind. Hunger is a sensation aroused by the rubbing together of the stomach walls when the stomach, being ready for food, begins its churning movements. Careful studies of sensations from the internal organs reveal astonishingly little of sensation arising there, but there can be little doubt that the sensations just listed really arise where they seem to arise, in the interior of the trunk.

Little has been done to determine the elementary sensations in this field; probably the organic sensations that every one is familiar with are blends rather than elements.