CHAPTER XX - PRODUCTION OF SENSATIONS
Our study of the nervous system has shown that impulses arising at the surface of the body are able, through connecting neurons, to bring about various activities. Moving along definite pathways, they induce motion in the muscles, and in the glands the secretion of liquids. It is now our purpose to consider the effect produced by afferent impulses upon the brain and, through the brain, upon the mind.114 This effect is manifested in a variety of similar forms, known as
The Sensations.—Sensations constitute the lowest forms of mental activity. Roughly speaking, they are the states of mind experienced as the direct result of impulses reaching the brain. In a sense, just as impulses passing to the muscles cause motion, impulses passing to the brain cause sensations. The feeling which results from the hand's touching a table is a sensation and so also is the pain which is caused by an injury to the body. The mental action in each case is due to impulses passing to the brain. Care must be exercised by the beginner, however, not to confuse sensations with the nervous impulses, on the one hand, or with secondary mental effects, such as emotion or imagination, on the other. Sensations are properly regarded as the first conscious effects of the afferent impulses and as the beginning stage in the series of mental processes that may take place on account of them.
[pg 339]In some way, not understood, the mind associates the sensation with the part of the body from which the impulses come. Pain, for example, is not felt at the brain where the sensation is produced, but at the place where the injury occurs. This association, by the mind, of the sensations with different parts of the body, is known as "localizing the sensation."
Sensation Stimuli.—While the sensations are dependent upon the afferent impulses, the afferent impulses are in turn dependent upon causes outside of the nervous system. If these are removed, the sensations cease and they do not start up again unless the exciting influences are again applied. Any agency, such as heat or pressure, which, by acting on the neurons of the body, is able to produce a sensation, may be called a sensation stimulus. It has perhaps already been observed that the stimuli that lead to voluntary action, as well as those that produce reflex action of the muscles, cause sensations at the same time. From this we may conclude that sensation stimuli are the same in character as those that excite motion. On the other hand, it should be noted that sensations are constantly resulting from stimuli that are of too mild a nature to cause motion.
Classes of Sensations.—Perhaps as many as twenty distinct sensations, such as pain, hunger, touch, etc., are recognized. If these are studied with reference to their origin, it will be seen that some of them result from the action of definite forms of stimuli upon the neurons terminating in sense organs; while the others, as a rule, arise from the action of indefinite stimuli upon neurons in parts of the body that do not possess sense organs. The members of the first class—and these include the sensations of touch, temperature, taste, smell, hearing, and sight—are[pg 340] known as the special sensations. The others, including the sensations of pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of disease, are known as organic, or general, sensations. These two classes of sensations differ in their purpose in the body as well as in the manner of their origin.
Purposes of Sensations.—Any given sensation is related to the stimulus which excites it as an effect to a cause. It starts up or stops, increases in intensity or diminishes, according to the action of the exciting stimulus. As the stimuli are outside of the nervous system, and in the majority of cases outside of the body, the sensations indicate to the mind what is taking place either in the body itself or in its surroundings. They supply, in other words, the means through which the mind acquires information. By means of the special sensations, a knowledge of the physical surroundings of the body is gained, and through the organic sensations the needs of the body and the state of the various organs are indicated. In general, sensations are made to serve two great purposes in the body, as follows:
1. They provide the necessary conditions for intelligent and purposeful action on the part of the body.
2. They supply the basis for the higher mental activities, as perception, memory, thought, imagination, and emotion.
Intelligent action is impossible without a knowledge both of the bodily organs and of the body's surroundings. Protection and the regulation of the work of an organ necessitate a knowledge of its condition, while the adapting and adjusting of the body to its surroundings require a knowledge of what those surroundings are. The dependence of all the higher forms of mental activity upon sensations is recognized by psychologists and is easily[pg 341] demonstrated by a study of the manner in which we acquire knowledge. "Without sensation there can be no thought."
Steps in the Production of Sensations.—The steps in the production of sensations are not essentially different from those in the production of reflex action. First of all, external stimuli act upon the fiber terminations in the sense organs, or elsewhere, starting impulses in the neurons. These pass into the central nervous system and there excite neurons which in turn discharge impulses into the cerebrum. The result is to arouse an activity of the mind—a sensation. The steps in the production of any special sensation naturally involve the following parts:
1. A sense organ where the terminations of the neurons are acted upon by the stimulus.
2. A chain of neurons which connect the sense organ with the brain.
3. The part of the cerebrum which produces the sensation.
Sense Organs.—The sense organs are not parts of the afferent neurons, but are structures of various kinds, in which the neurons terminate. Their function is to enable the sensation stimuli to start the impulses. By directing, concentrating, or controlling the stimuli, the sense organs enable them to act to the best advantage upon the neurons. When it is recognized that such widely different forces as light waves, sound waves, heat, pressure, and odors are enabled by them to stimulate neurons, the importance of these organs becomes apparent. As would naturally be inferred, the construction of any sense organ has particular reference to the nature of the stimulus which it is to receive. This is most apparent in the sense organs of sight and hearing.
[pg 342]Simple Forms of Sense Organs.—The simplest form of a sense organ (if such it may be called) is one found among the various tissues. It consists of the terminal branches of nerve fibers which spread over a small area of cells, as a network or plexus. Such endings are numerous in the skin and muscles.
Next in order of complexity are the so-called end-bulbs. These consist of rounded, or elongated, connective tissue capsules, within which the nerve fibers terminate. On the inside the fibers lose their sheaths and divide into branches, which wind through the capsule. End-bulbs are abundant in the lining membrane of the eye, and are found also in the skin of the lips and in the tissues around the joints.
Slightly more complex than the end-bulbs are the touch corpuscles. These are elongated bulb-like bodies, having a length of about one three-hundredth of an inch, and occupying the papillæ of the skin (Fig. 144). They are composed mainly of connective tissue. Each corpuscle receives the termination of one or more nerve fibers. These, on entering, lose the medullary sheath and separate into a number of branches that penetrate the corpuscle in different directions.
Fig. 144—A touch corpuscle highly magnified. (See text.)
The largest of the simple forms of sense organs are bodies visible to the naked eye and called, from their discoverer Pacini, the Pacinian corpuscles. They lie along the course of nerves in many parts of the body, and have the general form of grains of wheat. (See Practical Work.) The Pacinian corpuscles are composed of connective tissue[pg 343] arranged in separate layers around a narrow central cavity called the core (Fig. 145). Within the core is the termination of a large nerve fiber. These corpuscles are found in the connective tissue beneath the skin, along tendons, around joints, and among the organs of the abdominal cavity.
Fig. 145—Pacinian corpuscle, magnified. A. Medullated nerve fiber. B. Axis cylinder terminating in small bulb at C. D. Concentric layers of connective tissue. E. Inner bulb.
The simple forms of sense organs have a more or less general distribution over the body, and are concerned in the production of at least three special sensations. These are touch, temperature, and the muscular sensation.
Touch, or feeling, is perhaps the simplest of the sensations. The sense organs employed are the touch corpuscles, and the external stimulus is some form of pressure or impact. Pressure applied to the skin, by acting on the fiber terminations in the corpuscles, starts the impulses that give rise to the sensation. The touch corpuscles render the fiber terminations so sensitive that the slightest pressure is able to arouse sensations of touch. It is found that a change of pressure, rather than pressure that is constant, is the active stimulus. That all parts of the skin are not equally sensitive to pressure, and that the mind does not interpret equally well the sensations from different parts, are facts easily demonstrated by experiment. (See Practical Work.)
The Temperature Sensation.—Temperature sensations,[pg 344] like those of touch, are limited almost entirely to the skin. They are of two kinds, and are designated as heat sensations and as cold sensations. Whether the sense organs for temperature are different from those of touch is not known. It is known, however, that the same corpuscles do not respond alike to heat, cold, and pressure.
A Change of Temperature, rather than any specific degree of heat or cold, is the active temperature stimulus. The sensation of warmth is obtained when the temperature of the skin is being raised, and of cold when it is being lowered. This explains why in going into a hallway from a heated room one receives a sensation of cold, while in coming into the same hallway from the outside air he receives a sensation of warmth. It is for the same reason that we are able to distinguish only the relative, not the actual, temperature of bodies.
Muscular Sensations.—These are sensations produced by impulses arising at the muscles. Such impulses originate at the fiber terminations which are found in both the muscles and their tendons. By muscular sensations one is conscious of the location of a contracting muscle and of the degree of its tension. They also make it possible to judge of the weight of objects.
Fig. 146—Sense organs of taste. A. Map of upper surface of tongue, showing on the left the different kinds of papillæ, 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 papillæ (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[pg 345] 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 pass 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.
[pg 346]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—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[pg 347] resemble closely the cell-bodies of neurons, and are thought to be such. The divisions of the olfactory nerve pass 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 constituents of the atmosphere. In this connection it has been likened to a sentinel standing guard over the air passages. 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[pg 348] 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[pg 349] 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 pass 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[pg 351] 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 classified 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 shells 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—Diagram illustrating 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 quantity of energy, which the bell in turn imparts to the air. This energy is in the sound waves and is communicated to the[pg 352] 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.
Reënforcement of Sound Waves.—The sound vibrations from small bodies are not infrequently reënforced 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 reënforcement 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 reënforcing 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.)
[pg 353]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
The Sound-producing Mechanism of the Body consists of the following parts:
1. Delicately arranged bodies that are easily set in vibration.
2. An arrangement for supplying the necessary force for making these bodies vibrate.
3. Contrivances for modifying the vibrating parts so as to produce changes in pitch and intensity.
4. Parts that reënforce the vibrations.
5. Organs by means of which the sounds are converted into the forms of speech.
The central organ in this complex mechanism is
The Larynx.—The larynx forms a part of the air passages, being a short tube at the upper end of the trachea. Mucous membrane lines the inside of it and muscles cover most of the outer surface. The framework[pg 354] is made of cartilage. At the top it is partly encircled by a small bone (the hyoid), and its opening into the pharynx is guarded by a flexible lid, called the epiglottis. The cartilage in its walls is in eight separate pieces, but the greater portion of the structure is formed of two pieces only. These are known as the thyroid cartilage and the cricoid cartilage (Fig. 149). Both can be felt in the throat—the thyroid as the projection known as "Adam's apple," and the cricoid as a broad ring just below.
Fig. 149—The larynx.—A. Outside view. B. Vertical section through larynx, showing inside. 1. Thyroid cartilage. 2. Cricoid cartilage. 3. Trachea. 4. Hyoid bone. 5. Epiglottis. 6. Vocal cord. 7. False vocal cord. 8. Lining of mucous membrane.
The thyroid cartilage consists of two V-shaped pieces, one on either side of the larynx, meeting at their points in front, and each terminating at the back in an upward and a downward projection. Between the back portions of the thyroid is a space equal to about one third of the circumference of the larynx. This is occupied by the greater portion of the cricoid cartilage. This cartilage [pg 355]has the general shape of a signet ring and is so placed that the part corresponding to the signet fits into the thyroid space, while the ring portion encircles the larynx just below the thyroid. Muscles and connective tissue pass from the thyroid to the cricoid cartilage at all places, save one on each side, where the downward projections of the thyroid form hinge joints with the cricoid. These joints permit of motion of either cartilage upon the other.
At the summit of the cricoid cartilage, on each side, is a small piece of triangular shape, called the arytenoid cartilage. Each arytenoid is movable on the cricoid and is connected with one end of a vocal cord.
Fig. 150—Vocal cords as seen from above. A. In producing sound, B. During quiet breathing.
The Vocal Cords are formed by two narrow strips of tissue which, connecting with the thyroid cartilage in front and the arytenoid cartilages behind, lie in folds of the mucous membrane. They have the general appearance of ridge-like projections from the sides of the larynx, but at their edges they are sharp and smooth. The open space between the cords is called the glottis. When sound is not being produced, the glottis is open and has a triangular form, due to the spreading apart of the arytenoid cartilages and the attached cords. But when sound is being produced, the glottis is almost completely closed by the cords. Above the vocal cords, and resembling them in [pg 356]appearance, are two other folds of membrane, called the false vocal cords (B, Fig. 149). The false cords do not produce sound, but they aid in the closing of the glottis.
How the Voice is Produced.—The voice is produced through the vibrations of the vocal cords. A special set of muscles draws the arytenoid cartilages toward each other, thereby bringing their edges very near and parallel to each other in the passage. At the same time other muscles act on the thyroid and cricoid cartilages to separate them at the top and give the cords the necessary tension. With the glottis now almost closed, blasts of air from the lungs strike the sharp edges of the cords and set them in vibration (Fig. 150). The vocal cords do not vibrate as strings, like the strings of a violin, but somewhat as reeds, similar to the reeds of a French harp or reed organ.
The location of the vocal cords in the air passages enables the lungs and the muscles of respiration to aid in the production of the voice. It is their function to supply the necessary force for setting the cords in vibration. The upper air passages (mouth, nostrils, and pharynx) supply resonance chambers for reënforcing the vibrations from the vocal cords, thereby greatly increasing their intensity. In ordinary breathing the vocal cords are in a relaxed condition against the sides of the larynx and are not acted upon by the air as it enters or leaves the lungs.
Pitch and Intensity of the Voice.—Changes in the pitch of the voice are caused mainly by variations in the tension of the cords, due to the movements of the thyroid and cricoid cartilages upon each other.118 In the production of tones of very high pitch, the vibrating portions of the cords[pg 357] are thought to be actually shortened by their margins being drawn into contact at the back. This raises the pitch in the same manner as does the shortening of the vibrating portion of a violin string.
The intensity, or loudness, of the voice is governed by the force with which the air is expelled from the lungs. The vibrations of the cords, however, are greatly reënforced by the peculiar structure of the upper air passages, as stated above.
Production of Speech.—The sounds that form our speech or language are produced by modifying the vibrations from the vocal cords. This is accomplished by "mouthing" the sounds from the larynx. The distinct sounds, or words, are usually complex in nature, being made up of two or more elementary sounds. These are classed either as vowels or consonants and are represented by the different letters of the alphabet. The vowel sounds are made with the mouth open and are more nearly the pure vibrations of the vocal cords. The consonants are modifications of the vocal cord vibrations produced by the tongue, teeth, lips, and throat.
Words and their Significance.—In the development of language certain ideas have become associated with certain sounds so that the hearing of these sounds suggests the ideas. Our words, therefore, consist of so many sound signals, each capable of arousing a definite idea in the mind. To talk is to express ideas through these signals, and to listen is to assume an attitude of mind such that the signals may be interpreted. In learning a language, both the sounds of the words and their associated ideas are[pg 358] mastered, this being necessary to their practical use in exchanging ideas. From spoken language man has advanced to written language, so that the sight of the written or printed word also arouses in the mind the associated idea.
THE EAR
The Ear is the sense organ which enables sound waves to so act upon afferent neurons as to excite impulses in them. The effect upon the mind which these impulses produce is known as the sensation of hearing. In the performance of its function the ear receives and transmits sound waves and also concentrates them upon a suitable exposure of nerve cells. It includes three parts—the external ear, the middle ear, and the internal ear.
External Ear.—The external ear consists of the part on the outside of the head called the pinna, or auricle, and the tube leading into the middle ear, called the auditory canal (Fig. 151). The pinna by its peculiar shape aids to some extent the entrance of sound waves into the auditory canal.119 It consists chiefly of cartilage. The auditory canal is a little more than an inch in length and one fourth of an inch in diameter, and is closed at its inner end by a thin, but important membrane, called
The Membrana Tympani.—This membrane consists of three thin layers. The outer layer is continuous with the lining of the auditory canal; the inner is a part of the lining of the middle ear; and the middle is a fine layer of connective tissue. Being thin and delicately poised, the membrana tympani is easily made to vibrate by the sound[pg 359] waves that enter the auditory canal. In this way it serves as a receiver of sound waves from the air. It also protects
Fig. 151—Diagram of section through the ear, showing relations of its various parts. (See text.)
The Middle Ear.—The middle ear, or tympanum,120 consists of an irregular cavity in the temporal bone which is lined with mucous membrane and filled with air. It is connected with the pharynx by a slender canal called the Eustachian tube. Extending across the middle ear and connecting with the membrana tympani on one side, and with a membrane closing a small passage to the internal ear on the other, is a tiny bridge formed of three small bones. These bones, named in their order from the membrana tympani, are the malleus, the incus, and the stapes (Fig. 151). Where the malleus joins the membrane is a small muscle whose contraction has the effect of tightening[pg 360] the membrane. The Eustachian tube admits air freely to the middle ear, providing in this way for an equality of atmospheric pressure on the two sides of the drum membrane. The bridge of bones and the air in the middle ear receive vibrations from the membrana tympani and communicate them to the membrane of the internal ear.
Purposes of the Middle Ear. —The middle ear serves two important purposes. In the first place, it makes it possible for sound waves to set the membrana tympani in vibration. This membrane could not be made to vibrate by the more delicate of the sound waves if it were stretched over a bone, or over some of the softer tissues, or over a liquid. Its vibration is made possible by the presence of air on both sides, and this condition is supplied, on the inner side, by the middle ear. The Eustachian tube, by providing for an equality of pressure on the two sides of the membrane, also aids in this purpose.
In the second place, the middle ear provides a means for concentrating the force of the sound waves as they pass from the membrana tympani to the internal ear. This concentration is effected in the following manner:
1. The bridge of bones, being pivoted at one point to the walls of the middle ear, forms a lever in which the malleus is the long arm, and the incus and stapes the short arm, their ratio being about that of three to two. This causes the incus to move through a shorter distance, but with greater force than the end of the malleus.
2. The area of the membrana tympani is about twenty times as great as the membrane of the internal ear which is acted upon by the stapes. The force from the larger surface is, therefore, concentrated by the bridge of bones upon the smaller surface. By the combination of these two devices, the waves striking upon the membrane of the internal ear are rendered some thirty times more effective than are the same waves entering the auditory canal.
The Internal Ear, or labyrinth, occupies a series of irregular channels in the petrous process of the temporal bone.121 It is very complicated in structure, and at the same time is very small. Its greatest length is not more[pg 361] than three fourths of an inch and its greatest diameter not more than one half of an inch. It is filled with a liquid which at one place is called the perilymph, and at another place the endolymph. It is a double organ, being made up of an outer portion which lies next to the bone, and which surrounds an inner portion of the same general form. The outer portion is surrounded by a membrane which serves as periosteum to the bone and, at the same time, holds the liquid belonging to this part, called the perilymph. The inner portion, called the membranous labyrinth, consists essentially of a closed membranous sac, which is filled with the endolymph. The auditory nerve terminates in this portion of the internal ear. Three distinct divisions of the labyrinth have been made out, known as the vestibule, the semicircular canals, and the cochlea (Fig. 152).
Fig. 152—General form, of internal ear. The illustration represents the structures of the internal ear surrounded by a thin layer of bone. 1. Vestibule. 2. Cochlea. 3. Semicircular canals. 4. Fenestra ovalis. 5. Fenestra rotunda.
The Vestibule forms the central portion of the internal ear and is somewhat oval in shape. It is in communication with the middle ear through a small opening in the bone, called the fenestra ovalis, at which place it is separated from the middle ear only by a thin membrane. Sound waves enter the liquids of the internal ear at this point, the foot of the stapes being attached to the membrane. [pg 362]Six other openings lead off from the vestibule at different places. One of these enters the cochlea. The other five open into
The Semicircular Canals.—These canals, three in number, pass through the bone in three different planes. One extends in a horizontal direction and the other two vertically, but each plane is at right angles to the other two. Both ends of each canal connect with the vestibule, though two of them join by a common opening. The inner membranous labyrinth is continuous through each canal, and is held in position by small strips of connective tissue.
The purpose of the semicircular canals is not understood. It is known, however, that they are not used in hearing. On the other hand, there is evidence to the effect that they act as equilibrium sense organs, exciting sensations necessary for balancing the body. Their removal or injury, while having no effect upon the hearing, does interfere with the ability to keep the body in an upright position.
Fig. 153—Diagram showing the divisions of cochlear canal.
The Cochlea is the part of the internal ear directly concerned in hearing. It consists of a coiled tube which makes two and one half turns around a central axis and bears a close resemblance to a snail shell (Figs. 151 and 152). It differs in plan from a snail shell, however, in that its interior space is divided into three distinct channels, or canals. These lie side by side and are named, from their relations to other parts, the scala vestibula, the scala tympani, and the scala media. Any vertical section of the cochlea shows all three of these channels (Fig. 153).
[pg 363]The Scala Vestibula and the Scala Tympani appear in cross section as the larger of the canals. The former, so named from its connection with the vestibule, occupies the upper position in all parts of the coil. The latter lies below at all places, and is separated from the channels above partly by a margin of bone and partly by a membrane. It receives its name from its termination at the tympanum, or middle ear, from which it is separated only by a thin membrane.122 Both the scala vestibula and the scala tympani belong to the outer portion of the internal ear and are, for this reason, filled with the perilymph. At their upper ends they communicate with each other by a small opening, making by this means one continuous canal through the cochlea. This canal passes from the vestibule to the tympanum and, in so doing, goes entirely around
The Scala Media.—This division of the cochlea lies parallel to and between the other two divisions. It is above the scala tympani and below the scala vestibula, and is separated from each by a membrane. The scala media belongs to the membranous portion of the internal ear and is, therefore, filled with the endolymph. It receives the terminations of fibers from the auditory nerve and may be regarded as the true sense organ of hearing. The nerve fibers terminate upon the membrane known as the basilar membrane, which separates it from the scala tympani. This membrane extends the length of the cochlear canals, and is stretched between a projecting shelf of bone on one side and the outer wall of the cochlea on the other. It is covered with a layer of epithelial cells, some of which have small, hair-like projections and are known as the hair cells. Above the membrane, and resting partly upon it, are two[pg 364] rows of rod-like bodies, called the rods of Corti. These, by leaning toward each other, form a kind of tunnel beneath. They are exceedingly numerous, numbering more than 6000, and form a continuous series along the margin of the membrane.
Fig. 154—Diagram illustrating passage of sound waves through the ear.
How We Hear.—The sound waves which originate in vibrating bodies are transmitted by the air to the external ear. Passing through the auditory canal, the waves strike against the membrana tympani, setting it into vibration. By the bridge of bones and the air within the middle ear the vibrations are carried to and concentrated upon the liquid in the internal ear (Fig. 154). From here the vibrations pass through the channels of the cochlea and set into vibration the contents of the scala media and different portions of the basilar membrane. This serves as a stimulus to the fibers of the auditory nerve, causing them to transmit impulses which, on passing to the brain, produce the sensation of hearing.
Much of the peculiar structure of the cochlea is not understood. Its minute size and its location in the temporal bone make its study extremely difficult. The connection of the scala vestibula with the scala tympani, and this with the middle ear, is necessary for the passage of vibrations through the internal ear. Its liquids, being practically incompressible and surrounded on all sides by bones, could not otherwise yield to the movements of the stapes. (See Practical Work.) The rods of Corti are thought to act as dampers on the basilar membrane, to prevent the continuance of vibrations when once they are started.
Detection of Pitch.—The method of detecting tones of different pitch[pg 365] is not understood. Several theories have been advanced with reference to its explanation, one of the most interesting being that proposed by Helmholtz. This theory is based on our knowledge of sympathetic vibrations. The basilar membrane, while continuous throughout, may be regarded as made up of many separate cords of different lengths stretched side by side. A tone of a given pitch will set into vibration only certain of these cords, while tones of different pitch will set others into vibration.
Another theory is that the basilar membrane responds to all kinds of vibrations and the analysis of sound takes place in the brain.
A third view is that the filaments from the hair cells, rather than the basilar membrane, respond to the vibrations and in turn stimulate the terminations of the nerve fibers.
Fig. 155—Diagram showing how wax may plug the auditory canal and cause deafness.
Hygiene of the Ear.—The ear, being a delicate organ, is frequently injured by careless or rough treatment. The removal of the ear wax by the insertion of pointed instruments has been found to interfere with the natural method of discharge and to irritate the membrane. It should never be practiced. It is unnecessary in the healthy ear thus to cleanse the auditory canal, as the wax is passed by a natural process to where it is easily removed by a damp cloth. If the natural process is obstructed, clean warm water and a soft linen cloth may be employed in cleansing the canal, without likelihood of injury. Clean warm water may also be introduced into the auditory canal as a harmless remedy in relieving inflammation of the auditory canal and of the middle ear. Children's ears are easily injured, and it goes without saying that they should never be pulled nor boxed.
It frequently happens that a mass of wax collects in the auditory canal and closes the passage so completely as to[pg 366] cause deafness (Fig. 155). This may come about without pain and so gradually that one does not think of seeking medical aid. Such masses are easily removed by the physician, the hearing being then restored. Both for painful disturbances of the ear and for the gradual loss of hearing, the physician should be consulted.
The Hearing of School Children.—School children not infrequently have defective hearing and for this reason are slow to learn. The hearing is easily tested with a watch, the normal ear being able to hear the watch tick at a distance of at least two feet. Pupils with defective hearing should, of course, have medical attention, and in the classroom should be seated where they can hear to the best advantage.
Summary.—Sound waves constitute the external stimuli for the sensation of hearing. They consist of progressive vibratory movements of the air that originate in vibrating bodies. Through the larynx and the ear, sound waves are utilized by the body in different ways, but chiefly as a means of communication. The larynx produces sound waves which are reënforced and modified by the air passages. The ear supplies suitable conditions for the action of sound waves upon nerve cells. Both the ear and the larynx are constructed with special reference to the nature and properties of sound waves, and they illustrate the body's ability to adjust itself to, and to make use of, its physical environment.
Exercises.—1. For what different purposes are sound waves employed in the body?
2. How do sound waves originate? How are they transmitted? How do they differ from the waves on water?
3. How are sound waves able to act as nerve stimuli?
4. Describe two methods of reënforcing sound waves. Which method is employed in the body?
[pg 367]5. Name all the parts of the body that are directly or indirectly concerned in the production of sound.
6. Describe the larynx.
7. Describe the condition of the vocal cords in speaking and in ordinary breathing.
8. How are sounds differing in pitch and intensity produced by the larynx?
9. How is the sound produced by the vocal cords changed into speech?
10. What parts of the ear are concerned in transmitting sound waves?
11. Give the purposes of the middle ear.
12. Trace a sound wave from a bell to the basilar membrane, and trace the impulse that it causes from there to the brain.
13. Give the purpose of the Eustachian tubes; of the rods of Corti; of the semicircular canals.
14. Give directions for the proper care of the ear.
PRACTICAL WORK
To illustrate the Origin of Sound.—1. Strike a bell an easy blow and hold some light substance, as a pith ball attached to a thread, against the side, noting the result. 2. Sound a tuning fork by striking it against the table. Test it for vibrations as above, or by letting the vibrating prongs touch the surface of water. 3. Pluck a string of a guitar or violin, and find proof that it is vibrating while giving out sound.
To show the Transmission of Sound.—1. Vibrate a tuning fork and press the stem against a table or desk. The vibrations which are reënforced in this way will be heard in all parts of the room. Now press one end of a wooden rod, as a broom handle, against the table, and bring the stem of the vibrating fork against the other end. The vibrations now move down the stick to the table, from whence they are communicated to the air. Observe that the sound waves, to reach the ear, must pass through the rod, the table, and the air. 2. Fasten the tuning fork to a flat piece of cork by pressing the stem into a small hole in the center. Vibrate the fork and let the cork rest on the surface of water in a half-filled tumbler on the table. The sound will, as before, pass to the table and then to the air. Observe that in this case the vibrations are transmitted by a liquid, a solid, and by the air.[pg 368] Compare this action with the transmission of sound waves by different portions of the ear.
To show Effects of Sound Waves.—1. Place two large tuning forks of the same pitch, and mounted on thin boxes for reënforcing their vibrations, near each other on a table. Vibrate one of the forks for a moment and then stop it by means of the hand. Observe that the other fork has been set in vibration. (This experiment does not work with forks of different pitch.) 2. While holding a thin piece of paper against a comb with the open lips, produce musical tones with the vocal cords. These will set the paper in vibration, producing the so-called "comb music." 3. Examine the disk in a telephone which is set in vibration by the voice. Observe that it is a thin disk and, like the membrane of the ear, has air on both sides of it.
To show the Reënforcement of Sound.—1. Vibrate a tuning fork in the air, noting the feebleness of the tone produced. Then hold the stem against a door or the top of a table, noting the difference. 2. Hold a vibrating tuning fork over a tall jar, or bottle, and gradually add water. If the vessel is sufficiently tall, a depth will be reached where the air in the vessel reënforces the sound from the fork. 3. Hold a vibrating fork over the mouth of a small fruit jar, partly covered with a piece of cardboard. By varying the size of the opening, a position will be found where the sound is reënforced. If not successful at first, try bottles and jars of different sizes.
To illustrate the Manner of Vibration of the Liquid in the Internal Ear.—Tie a piece of dental rubber over the end of a glass or wooden tube about half an inch in diameter and six inches in length. Fill the tube entirely full of water and, without spilling, tie a piece of thin rubber tightly over the other end. Holding the tube horizontally, press the rubber in at one end and note that it is pushed out at the other end. Make an imitation of a vibration with the finger against the rubber at one end of the tube and note the effect at the other end. To what do the tube and the rubber on the ends of the tube correspond in the internal ear?
Fig. 156—Simple apparatus for demonstrating the larynx.
To show the Plan of the Larynx.—Cut from stiff paper four pieces of different shapes as indicated in Fig. 156. (The piece to the left should have a length of about six inches, the others proportionally[pg 369] large.) The largest represents the thyroid cartilage, the next in size the cricoid, and the two smallest the arytenoid cartilages. By means of pins, or threads, connect these with each other according to the description of the larynx on page 253. With this simple model the movements of the different cartilages and their effect upon the vocal cords may be illustrated.
To show the Relation of the Movements of the Vocal Organs to the Production of Different Sounds.—1. Lightly grasp the larynx with the fingers while talking. Observe the changes, both in the position and shape of the larynx, in the production of sounds of different pitch. 2. Observe the difference in the action of the muscles of respiration in the production of loud and faint sounds. 3. Pronounce slowly the vowels, A, E, I, O, U, and the consonants C, F, K, M, R, S, T, and V, noting the shape of the mouth, the position of the tongue, and the action of the lips in each case.
To demonstrate the Ear.—Examine a dissectible model of the ear, locating and naming the different parts. Trace as far as possible the path of the sound waves and find the termination of the auditory nerve. Note also the relative size of the parts, and calculate the number of times the model is larger than the natural ear. Suggestion: The greatest diameter of the internal ear is about three fourths of an inch.
In an extended course it is a profitable exercise to dissect the ear of a sheep or calf, observing the auditory canal, middle ear, bridge of bones, and the tympanic membrane with attached malleus and tensor tympanic muscle. Pass a probe from the nasal pharynx through the Eustachian tube into the middle ear. With bone forceps or a fine saw, split open the petrous portion of the temporal bone and observe the cochlea and the semicircular canals. By a careful dissection other parts of interest may also be shown.