Fig. 146-*Sense organs of taste.* _A._ Map of upper surface of tongue, showing on the left the different kinds of papillae, and on the right the areas of taste (after Hall). Area sensitive to bitter (--); to acid (....); to salt (-.-.-.-); to sweet (----). _B._ Section through a papilla. _n._ Small nerve connecting with taste buds at _d. e._ Epithelium. _C._ Single taste bud magnified. _n._ Nerve, the fibers of which terminate between the spindle-shaped cells _a. e._ Epithelial cells.

*The Sensation of Taste.*-The sense organs of taste are found chiefly in the mucous membrane covering the upper surface of the tongue. Scattered over this surface are a number of rounded elevations, or large papillae (A, Fig. 146). Toward the back of the tongue two rows of these, larger than the others, converge to meet at an angle, where is located a papilla of exceptional size. Surrounding each papilla is a narrow depression, within which are found the sense organs of taste (B, Fig. 146). These are called, from their shape, _taste buds_, and each bud contains a central cavity which communicates with the surface by a small opening-_the gustatory pore_. Within this cavity are many slender, spindle-shaped cells which terminate in hair-like projections at the end nearest the pore, but in short fibers at the other end. Nerve fibers enter at the inner ends of the buds and spread out between the cells (_C_, Fig. 146). These fibers pa.s.s to the brain as parts of two pairs of nerves-those from the front of the tongue joining the trigeminal nerve, and those from the back of the tongue, the glossopharyngeal nerve.

The gustatary, or _taste stimulus_, is some chemical or physical condition of substances which is manifested only when they are in a liquid state.

For this reason _only liquid substances can be tasted_. Solids to be tasted must first be dissolved.

The different taste sensations are described as bitter, sweet, sour, and saline, and in the order named are recognized as the tastes of quinine, sugar, vinegar, and salt. As to how these different tastes are produced, little is known. Flavors such as vanilla and lemon, and the flavors of meats and fruits, are really smelled and not tasted. Taste serves two main purposes: it is an aid in the selection of food and it is a means of stimulating the digestive glands (page 161).

[Fig. 147]

Fig. 147-*Sense organ of smell.* _A._ Distribution of nerves in outer wall of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves.

3. Branches of olfactory nerve. 4. Olfactory bulb. _B._ Diagram showing connection of neurons concerned in smell.

*The Sensation of Smell.*-The sense organs of smell are found in the mucous membrane lining the upper divisions of the nasal cavities. Here are found two kinds of cells in great abundance-column-shaped epithelial cells and the cells which are recognized as the sense organs of smell. These olfactory cells are spindle-shaped, having at one end a slender, thread-like projection which reaches the surface, and at the other end a fiber which joins an olfactory nerve (B, Fig. 147). In fact, the olfactory cells resemble closely the cell-bodies of neurons, and are thought to be such. The divisions of the olfactory nerve pa.s.s through many small openings in the ethmoid bone to connect with the olfactory bulbs, which in turn connect with the cerebrum (A, Fig. 147).

*The Olfactory Stimulus.*-Only substances in the gaseous state can be smelled. From this it is inferred that the stimulus is supplied by gas particles. Solids and liquids are smelled because of the gas particles which separate from them. The substance which is smelled must be kept moving through the nostrils and made to come in direct contact with the olfactory cells. There is practically no limit to the number of distinct odors that may be recognized.

*Value of Smell.*-Although the sense of smell is not so acute in man as in some of the lower animals, it is, nevertheless, a most important and useful gift. It is the only sense that responds to matter in the gaseous state, and is, for this reason, the only natural means of detecting harmful const.i.tuents of the atmosphere. In this connection it has been likened to a sentinel standing guard over the air pa.s.sages. Many gases are, however, without odor, and for this reason cannot be detected by the nostrils. It is of especial importance that gases which are likely to become mixed with the air supply to the body have odor, even though the odor be disagreeable. The bad odors of illuminating gas and of various compounds of the chemical laboratory, since they serve as danger signals to put one exposed to them on his guard, are of great protective value.

*Sight and Hearing.*-The sense organs of sight and hearing are highly complicated structures, and will be considered in the chapters following.

*Summary.*-Sensations are certain activities of the mind that result from excitations within the body or at its surface. These cause the neurons to discharge impulses which on reaching the cerebrum cause the sensations.

Sensations are necessary for intelligent and purposeful action and for acquiring all kinds of knowledge. To enable the stimuli to act to the best advantage in starting the impulses, special devices, called sense organs, are employed. These receive the terminations of the neurons, and by their special structure enable the most delicate stimuli to start impulses. The simpler forms of sense organs are those of touch, temperature, taste, and smell.

*Exercises.*-1. Compare sensations and reflex actions with reference to their nature and cause. Give steps in the production of each.

2. Give examples of sensation stimuli. State the purpose of sense organs.

3. How do general sensations differ from special sensations?

4. Of what value is pain in the protection of the body?

5. Show that sensations lead to the higher forms of mental activity, such as emotion and imagination.

6. Of what value to the body is the "localizing of the sensation"?

7. What kinds of sense organs are found in the skin? State the purpose of each.

8. Through what sense avenues is one made aware of solids, of liquids, and of gases?

9. Of what special protective value is the sense of smell?

PRACTICAL WORK

*To demonstrate the Pacinian Corpuscles.*-Spread out the mesentery from the intestine of a cat and hold it between the eye and the light: Pacinian corpuscles will appear as small translucent bodies having the general form of grains of wheat. Secure a portion of the mesentery over a circular opening in a thin piece of cork and examine it with a microscope of low power. Follow the course of the nerve fiber to the nerve from which it branches.

*To show Relative Sensitiveness of Different Parts of the Skin.*-Holding a bristle between the fingers, bring the end in contact with the skin, noting the amount of pressure necessary to cause a sensation of touch.

Test the lips, tongue, tips of fingers, and palm and back of hand, trying different sizes of bristles. Has the degree of sensitiveness any relation to the thickness of the cuticle?

*To show Perceptive Differences of Different Portions of the Skin.*-Place the points of a pair of dividers on the back of the hand of one who looks in the opposite direction. Is one point felt or two? Repeat several times, changing the distance between the points until it is fully determined how near the two points must be placed in order to be felt as one. In like manner test other parts of the body, as the tips of the fingers and the back of the neck. Compare results obtained at different places.

*To locate Warm and Cold Sensation Spots.*-Slowly and evenly draw a blunt-pointed piece of metal over the back of the neck. If it be of the same temperature as the skin, only touch sensations will be experienced.

If it be a little colder (the temperature of the room) sensations of cold will be felt at certain spots. If slightly warmer than the body, heat sensation spots will be found on other parts of the skin. If the heat and cold sensation spots be marked and tested from day to day they will be found to remain constant as to position. Inference.

CHAPTER XXI - THE LARYNX AND THE EAR

Man is a social being. His inclinations are not to live alone, but to be a part of that great human organization known as society. For men to work together, to be mutually helpful one to another, requires the ability to exchange ideas and this in turn requires some means of communication.(115) One means of communication is found in certain movements of the atmosphere, known as _sound waves_. In the exchange of ideas by this means there are employed two of the most interesting divisions of the body-the larynx and the ear. The first is an instrument for the production of sound waves; the second is the sense organ which enables the sound waves to act as stimuli to the nervous system.

*Nature of Sound Waves.*-If some sonorous body, as a bell, be struck, it is given a quivering, or vibratory, motion. This is not confined to the bell, but is imparted to the air and other substances with which the bell comes in contact. These take up the movements and pa.s.s them to objects more remote, and they in turn give them to others, until a very considerable distance is reached. Such progressive vibrations are known as waves, and, since they act as stimuli to the organs of hearing, they are called _sound waves_. Sound waves _always originate in vibrating bodies_.(116) They are transmitted chiefly _by the air_, which, because of its lightness, elasticity, and abundance, readily takes up the vibrations and spreads them in all directions (Fig. 148).

While these vibratory movements of the atmosphere are correctly cla.s.sified as waves, they bear little resemblance to the waves on water. Instead of being made of crests and troughs, as are the water waves, the sound waves consist of alternating successions of slightly condensed and rarefied layers of air. Then, while the general movement of the water waves is that of ever widening circles _over a surface_, the sound waves spread as enlarging spherical sh.e.l.ls _through_ the air. In sound waves, as in all other waves, however, it is only the form of the wave that moves forward.

The individual particles of air that make up the wave simply vibrate back and forth.

[Fig. 148]

Fig. 148-Diagram ill.u.s.trating the spreading of sound waves through air.

*How Sound Waves act as Stimuli.*-Any sound wave represents a small but definite amount of energy, this being a part of the original force that acted on the vibrating body to set it in motion. The hammer, for instance, in striking a bell imparts to it a measurable quant.i.ty of energy, which the bell in turn imparts to the air. This energy is in the sound waves and is communicated to the bodies against which they strike.(117) Though the force exerted by most sound waves is, indeed, very slight, it is sufficient to enable them to act as stimuli to the nervous system.

*How Sounds Differ.*-Three distinct effects are produced by sound waves upon the nerves of hearing, and through them upon the mind. These are known as _pitch, intensity_, and _quality_, and they are dependent upon the vibrations of the sound-producing bodies.

_Pitch_, which has reference to the height, or degree of sharpness, of tones, is determined by the rapidity of the vibrations of the vibrating body. The more rapid the vibrations, the higher the pitch, the number of vibrations doubling for each musical interval known as the octave.

_Intensity_ is the energy, or force, of the sound waves. This is recognized by the strength of the sensation and is expressed by the term _loudness_. Intensity is governed mainly by the width of the vibrations of the vibrating body, and the width depends upon the force applied to the body to make it vibrate.

_Quality_ is that peculiarity of sound that enables tones from different instruments to sound differently, although they may have the same pitch and intensity. Quality depends upon the fact that most tones are complex in nature and result from the blending together of simple tones of different pitch.

*Reenforcement of Sound Waves.*-The sound vibrations from small bodies are not infrequently reenforced by surrounding conditions so that their outgoing waves reach farther and are more effective than waves from larger bodies. This is true of the sound waves produced by most musical instruments and also those produced by the human larynx. Such reenforcement is effected in two general ways-by sounding boards and by inclosed columns of air. Stringed instruments-violin, guitar, piano, etc.-employ sounding boards, while wind instruments, as the flute, pipe organ, and the various kinds of horns, employ air columns for reenforcing their vibrations. In the use of the sounding board, the vibrations are communicated to a larger surface, and in the use of the air column the vibrations are communicated to the inclosed air. (See Practical Work.)

*Value of Sound Waves to the Body.*-From a physiological standpoint, the value of sound waves is not easily overestimated. In addition to the use made of them in the communication of ideas, they serve the purpose of protecting the body, and in the sphere of music provide one of the most elevating forms of entertainment. Sounds from different animals, as well as from inanimate objects, may also be the means of supplying needed information. The existence of two kinds of sound instruments in the body-the one for the production, the other for the detection, of sound-is certainly suggestive of the ability of the body to adjust itself to, and to make use of, its physical environment. Both the larynx and the ear are constructed with special reference to the nature and properties of sound waves.

THE LARYNX