Additional Experiments.

Experiment 162. Use a small pair of wooden compasses, or an ordinary pair of dividers with their points guarded by a small piece of cork. Apply the points of the compasses lightly and simultaneously to different parts of the body, and ascertain at what distance apart the points are felt as two. The following is the order of sensibility: tip of tongue, tip of the middle finger, palm, forehead, and back of hand.

Experiment 163. Test as in preceding experiment the skin of the arm, beginning at the shoulder and passing downwards. Observe that the sensibility is greater as one tests towards the fingers, and also in the transverse than in the long axis of the limb. In all cases compare the results obtained on both sides of the body.

Experiment 164. By means of a spray-producer, spray the back of the hand with ether, and observe how the sensibility is abolished.

Experiment 165. Touch your forehead with your forefinger; the finger appears to feel the contact, but on rubbing the forefinger rapidly over the forehead, it is the latter which is interpreted as “feeling” the finger.

Experiment 166. Generally speaking, the sensation of touch is referred to the cutaneous surfaces. In certain cases, however, it is referred even beyond this. Holding firmly in one hand a cane or a pencil, touch an object therewith; the sensation is referred to the extremity of the cane or pencil.
If, however, the cane or pencil be held loosely in one’s hand, one experiences two sensations: one corresponding to the object touched, and the other due to the contact of the rod with the skin. The process of mastication affords a good example of the reference of sensations to and beyond the periphery of the body.

Experiment 167. Prepare a strong solution of sulphate of quinine with the aid of a little sulphuric acid to dissolve it (bitter), a five-per-cent solution of sugar (sweet), a ten-per-cent solution of common salt (saline), and a one-per-cent solution of acetic acid (acid). Wipe the tongue dry, and lay on its tip a crystal of sugar. It is not tasted until it is dissolved.

Experiment 168. Apply a crystal of sugar to the tip, and another to the back of the tongue. The sweet taste is more pronounced at the tip.

Experiment 169. Repeat the process with sulphate of quinine in solution. It is scarcely tasted on the tip, but is tasted immediately on the back part of the tongue. Test where salines and acids are tasted most acutely.

Experiment 170. To illustrate the muscular sense. Take two equal iron or lead weights; heat one and leave the other cold. The cold weight will feel the heavier.

Experiment 171. Place a thin disk of cold lead, the size of a silver dollar, on the forehead of a person whose eyes are closed; remove the disk, and on the same spot place two warm disks of equal size. The person will judge the latter to be about the same weight, or lighter, than the single cold disk.

Experiment 172. Compare two similar wooden disks, and let the diameter of one be slightly greater than that of the other. Heat the smaller one to over 120° F., and it will be judged heavier than the larger cold one.

Experiment 173. To illustrate the influence of excitation of one sense organ on the other sense organs. Small colored patches the shape and color of which are not distinctly visible may become so when a tuning-fork is kept vibrating near the ears. In other individuals the visual impressions are diminished by the same process.
On listening to the ticking of a watch, the ticking sounds feebler or louder on looking at a source of light through glasses of different colors.
If the finger be placed in cold or warm water the temperature appears to rise when a red glass is held in front of the eyes.

Experiment 174. Formation of an inverted image on the retina. Take a freshly removed ox-eye; dissect the sclerotic from that part of its posterior segment near the optic nerve. Roll up a piece of blackened paper in the form of a tube, black surface innermost, and place the eye in it with the cornea directed forward. Look at an object—e.g., a candle-flame—and observe the inverted image of the flame shining through the retina and choroid, and notice how the image moves when the candle is moved.

Experiment 175. Focus a candle-flame or other object on the ground-glass plate of an ordinary photographic camera, and observe the small inverted image.

Experiment 176. To illustrate spherical aberration. Make a pin-hole in a blackened piece of cardboard; look at a light placed at a greater distance than the normal distance of accommodation. One will see a radiate figure with four to eight radii. The figures obtained from opposite eyes will probably differ in shape.

Experiment 177. Hold a thin wooden rod or pencil about a foot from the eyes and look at a distant object. Note that the object appears double. Close the right eye; the left image disappears, and vice versa.

Experiment 178. To show the movements of the iris. It is an extremely beautiful experiment, and one that can easily be made. Look through a pin-hole in a card at a uniform white surface as the white shade of an ordinary reading-lamp. With the right eye look through the pin-hole, the left eye being closed. Note the size of the (slightly dull) circular visual field. Open the left eye, the field becomes brighter and smaller (contraction of pupil); close the left eye, after an appreciable time, the field (now slightly dull) is seen gradually to expand. One can thus see and observe the rate of movements of his own iris.

Fig. 145.

Experiment 179. To show the blind spot. The left eye being shut, let the right eye be fixed upon the cross as in Fig. 145. When the book is held at arm’s length, both cross and round spot will be visible; but if the book be brought to about 8 inches from the eye, the gaze being kept steadily upon the cross, the round spot will at first disappear, but as the book, is brought still nearer both cross and round spot will again be seen.

Experiment 180. To illustrate the duration of retinal impressions. On a circular white disk, about halfway between the center and circumference, fix a small, black, oblong disk, and rapidly rotate it by means of a rotating wheel. There appears a ring of gray on the black, showing that the impression on the retina lasts a certain time.

Fig. 146.—Optic Disks.
The disk A, having black and white sectors, when rotated rapidly gives an even gray tint as in B.

Experiment 181. Mark off a round piece of cardboard into black and white sectors as in A ([Fig. 146]). Attach it so as to rotate it rapidly, as on a sewing machine. An even gray tint will be produced as in B.

Experiment 182.To illustrate imperfect visual judgments. Make three round black dots, A, B, C, of the same size, in the same line, and let A and C be equidistant from B. Between A and B make several more dots of the same size. A and B will then appear to be farther apart than B and C.

For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross-bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other.

Experiment 183. Make on a white card two squares of equal size. Across the one draw horizontal lines at equal distances, and in the other make similar vertical lines. Hold them at some distance. The one with horizontal lines appears higher than it really is, while the one with vertical lines appears broader, i.e., both appear oblong.

Experiment 184. Look at the row of letters (S) and figures (8). To some the upper halves of the letters and figures may appear to be of the same size as the lower halves, to others the lower halves may appear larger. Hold the figure upside down, and observe that there is a considerable difference between the two, the lower halves being considerably larger.

S S S S S S S S 8 8 8 8 8 8 8 8

Experiment 185. To illustrate imperfect visual judgment. The length of a line appears to vary according to the angle and direction of certain other lines in relation to it ([Fig. 147]). The length of the two vertical lines is the same, yet B appears much longer than A.

Fig. 147.—To show False Estimate of Size.

Experiment 186. In indirect vision the appreciation of direction is still more imperfect. While leaning on a large table, fix a point on the table, and then try to arrange three small pieces of colored paper in a straight line. Invariably, the papers, being at a distance from the fixation-point, and being seen by indirect vision, are arranged, not in a straight line, but in the arc of a circle with a long radius.

Chapter XII.
The Throat and the Voice.

349. The Throat. The throat is a double highway, as it were, through which the air we breathe traverses the larynx on its way to the lungs, and through which the food we swallow reaches the œsophagus on its passage to the stomach. It is, therefore, a very important region of the body, being concerned in the great acts of respiration and digestion.

The throat is enclosed and protected by various muscles and bony structures, along which run the great blood-vessels that supply the head, and the great nerve trunks that pass from the brain to the parts below.

We have already described the food passages ([Chapter VI].) and the air passages ([Chapter VIII.]).

To get a correct idea of the throat we should look into the wide-open mouth of some friend. Depressing the tongue we can readily see the back wall of the pharynx, which is common to the two main avenues leading to the lungs and the stomach. Above, we notice the air passages, which lead to the posterior cavities of the nose. We have already described the hard palate, the soft palate, the uvula, and the tonsils ([Fig. 46]).

On looking directly beyond these organs, we see the beginning of the downward passage,—the pharynx. If now the tongue be forcibly drawn forward, a curved ridge may be seen behind it. This is the epiglottis, which, as we have already learned shuts down, like the lid of a box, over the top of the larynx (secs. 137 and 203).

The throat is lined with mucous membrane covered with ciliated epithelium, which secretes a lubricating fluid which keeps the parts moist and pliable. An excess of this secretion forms a thick, tenacious mass of mucus, which irritates the passages and gives rise to efforts of hawking and coughing to get rid of it.

350. The Larynx. The larynx, the essential organ of voice, forms the box-like top of the windpipe. It is built of variously shaped cartilages, connected by ligaments. It is clothed on the outside with muscles; on the inside it is lined with mucous membrane, continuous with that of the other air passages.

Fig. 148.—View of the Cartilages in front project and form the lages and Ligaments of the “Adam’s apple,” plainly seen and Larynx. (Anterior view.)

The larynx has for a framework two cartilages, the thyroid and the cricoid, one above the other. The larger of these, called the thyroid, from a supposed resemblance to a shield, consists of two extended wings which join in front, but are separated by a wide interval behind. The united edges in front project and form the “Adam’s apple” plainly seen and easily felt on most people, especially on very lean men.

Above and from the sides rise two horns connected by bands to the hyoid bone from which the larynx is suspended. This bone is attached by muscles and ligaments to the skull. It lies at the base of the tongue, and can be readily felt by the finger behind the chin at the angle of the jaw and the neck (sec. 41 and Fig. 46). From the under side of the thyroid two horns project downwards to become jointed to the cricoid. The thyroid thus rests upon, and is movable on, the cricoid cartilage.

The cricoid cartilage, so called from its fancied resemblance to a signet-ring, is smaller but thicker and stronger than the thyroid, and forms the lower and back part of the cavity of the larynx. This cartilage is quite sensitive to pressure from the fingers, and is the cause of the sharp pain felt when we try to swallow a large and hard piece of food not properly chewed.

Fig. 149.—Diagram of a Sectional of Nasal and Throat Passages.

On the upper edge of the cricoid cartilage are perched a pair of very singular cartilages, pyramidal in shape, called the arytenoid, which are of great importance in the production of the voice. These cartilages are capped with little horn-like projections, and give attachment at their anterior angles to the true vocal cords, and at their posterior angles to the muscles which open and close the glottis, or upper opening of the windpipe. When in their natural position the arytenoid cartilages resemble somewhat the mouth of a pitcher, hence their name.

351. The Vocal Cords. The mucous membrane which lines the various cartilages of the larynx is thrown into several folds. Thus, one fold, the free edge of which is formed of a band of elastic fibers, passes horizontally outwards from each side towards the middle line, at the level of the base of the arytenoid cartilages. These folds are called the true vocal cords, by the movements of which the voice is produced.

Above them are other folds of mucous membrane called the false vocal cords, which take no part in the production of the voice. The arrangement of the true vocal cords, projecting as they do towards the middle line, reduces to a mere chink the space between the part of the larynx above them and the part below them. This constriction of the larynx is called the glottis.

Fig. 150.—View of the Cartilages and Ligaments of Larynx. (Posterior view.)

352. The Mechanism of the Voice. The mechanism of the voice may be more easily understood by a study of Fig. 150. We have here the larynx, viewed from behind, with all the soft parts in connection with it. On looking down, the folds forming the true vocal cords are seen enclosing a V-shaped aperture (the glottis), the narrow part being in front.

The form of this aperture may be changed by the delicately coordinate activities of the muscles of the larynx. For instance, the vocal cords may be brought so closely together that the space becomes a mere slit. Air forced through the slit will throw the edges of the folds into vibration and a sound will be produced.

The Variations in the form of the opening will determine the variations in the sound. Now, if the various muscles of the larynx be relaxed, the opening of the glottis is wider. Thus the air enters and leaves the larynx during breathing, without throwing the cords into vibration enough to produce any sound.

We may say that the production of the voice is effected by an arrangement like that of some musical instruments, the sounds produced by the vibrations of the vocal cords being modified by the tubes above and below. All musical sounds are due to movements or vibrations occurring with a certain regularity, and they differ in loudness, pitch, and quality. Loudness of the sound depends upon the extent of the vibrations, pitch on the rapidity of the vibrations, and quality on the admixture of tones produced by vibrations of varying rates of rapidity, related to one another.

Fig. 151.—Longitudinal Section of the Larynx. (Showing the vocal cords.)

353. Factors in the Production of the Voice. Muscles which pass from the cricoid cartilage to the outer angle of the arytenoids act to bring the vocal cords close together, and parallel to one another, so that the space between them is narrowed to a slit. A strong expiration now drives the air from the lungs through the slit, between the cords, and throws them into vibration. The vibration is small in amount, but very rapid. Other muscles are connected with the arytenoid cartilages which serve to seperate the vocal cords and to widely open the glottis. The force of the outgoing current of air determines the extent of the movement of the cords, and thus the loudness of the sound will increase with greater force of expiration.

We have just learned that the pitch of sound depends on the rapidity of the vibrations. This depends on the length of cords and their tightness for the shorter and tighter a string is, the higher is the note which its vibration produces. The vocal cords of women are about one-third shorter than those of men, hence the higher pitch of the notes they produce. In children the vocal cords are shorter than in adults.[[50]] The cords of tenor singers are also shorter than those of basses and baritones. The muscles within the larynx, of course, play a very important part in altering the tension of the vocal cords. Those qualities of the voice which we speak of as sweet, harsh, and sympathetic depend to a great extent upon the peculiar structure of the vocal cords of the individual.

Besides the physical condition of the vocal cords, as their degree of smoothness, elasticity, thickness, and so on, other factors determine the quality of an individual’s voice. Thus, the general shape and structure of the trachea, the larynx, the throat, and mouth all influence the quality of voice. In fact, the air passages, both below and above the vibrating cords, act as resonators, or resounding chambers, and intensify and modify the sounds produced by the cords. It is this fact that prompts skillful teachers of music and elocution to urge upon their pupils the necessity of the mouth being properly opened during speech, and especially during singing.

Experiment 187. To show the anatomy of the throat. Study the general construction of the throat by the help of a hand mirror. Repeat the same on the throat of some friend.

Experiment 188. To show the construction of the vocal organs. Get a butcher to furnish two windpipes from a sheep or a calf. They differ somewhat from the vocal organs of the human body, but will enable us to recognize the different parts which have been described, and thus to get a good idea of the gross anatomy.

One specimen should be cut open lengthwise in the middle line in front, and the other cut in the same way from behind.

354. Speech. Speech is to be distinguished from voice. It may exist without voice, as in a whisper. Speech consists of articulated sounds, produced by the action of various parts of the mouth, throat, and nose. Voice is common to most animals, but speech is the peculiar privilege of man.

Fig. 152.—Diagramatic Horizontal Section of Larynx to show the Direction of Pull of the Posterior Crico-Arytenoid Muscles, which abduct the Vocal Cords. (Dotted lines show position in abduction.)]

The organ of speech is perhaps the most delicate and perfect motor apparatus in the whole body. It has been calculated that upwards of 900 movements per minute can be made by the movable organs of speech during reading, speaking, and singing. It is said that no less than a hundred different muscles are called into action in talking. Each part of this delicate apparatus is so admirably adjusted to every other that all parts of this most complex machinery act in perfect harmony.

There are certain articulate sounds called vowel or vocal, from the fact that they are produced by the vocal cords, and are but slightly modified as they pass out of the mouth. The true vowels, a, e, i, o, u, can all be sounded alone, and may be prolonged in expiration. These are the sounds chiefly used in singing. The differences in their characters are produced by changes in the position of the tongue, mouth, and lips.

Consonants are sounds produced by interruptions of the outgoing current of air, but in some cases have no sound in themselves, and serve merely to modify vowel sounds. Thus, when the interruption to the outgoing current takes place by movements of the lips, we have the labial consonants, p, b, f, and v. When the tongue, in relation with the teeth or hard palate, obstructs the air, the dental consonants, d, t, l, and s are produced. Gutturals, such as k, g, ch, gh, and r, are due to the movements of the root of the tongue in connection with the soft palate or pharynx.

To secure an easy and proper production of articulate sounds, the mouth, teeth, lips, tongue, and palate should be in perfect order. The modifications in articulation occasioned by a defect in the palate, or in the uvula, by the loss of teeth, from disease, and from congenital defects, are sufficiently familiar. We have seen that speech consists essentially in a modification of the vocal sounds by the accessory organs, or by parts above the larynx, the latter being the essential vocal instrument.

Many animals have the power of making articulated sounds; a few have risen, like man, to the dignity of sentences, but these are only by imitation of the human voice. Both vowels and consonants can be distinguished in the notes of birds, the vocal powers of which are generally higher than those of mammals. The latter, as a rule, produce only vowels, though some are also able to form consonants.

Persons idiotic from birth are incapable of producing any other vocal sounds than inarticulate cries, although supplied with all the internal means of articulation. Persons deaf and dumb are in the same situation, though from a different cause; the one being incapable of imitating, and the other being deprived of hearing the sounds to be imitated.

Fig. 153.—Direction of Pull of the Lateral Crico-Arytenoids, which adduct the Vocal Cords. (Dotted lines show position in adduction.)

In whispering, the larynx takes scarcely any part in the production of the sounds; the vocal cords remain apart and comparatively slack, and the expiratory blast rushes through without setting them in vibration.

In stammering, spasmodic contraction of the diaphragm interrupts the effort of expiration. The stammerer has full control of the mechanism of articulation, but not of the expiratory blast. His larynx and his lips are at his command, but not his diaphragm. To conquer this defect he must train his muscles of respiration to calm and steady action during speech. The stutterer, on the other hand, has full control of the muscles of expiration. His diaphragm is well drilled, but his lips and tongue are insubordinate.

355. The Care of the Throat and Voice. The throat, exposed as it is to unwholesome and overheated air, irritating dust of the street, factories, and workshops, is often inflamed, resulting in that common ailment, sore throat. The parts are red, swollen, and quite painful on swallowing. Speech is often indistinct, but there is no hoarseness or cough unless the uvula is lengthened and tickles the back part of the tongue. Slight sore throat rarely requires any special treatment, aside from simple nursing.

The most frequent cause of throat trouble is the action of cold upon the heated body, especially during active perspiration. For this reason a cold bath should not be taken while a person is perspiring freely. The muscles of the throat are frequently overstrained by loud talking, screaming, shouting, or by reading aloud too much. People who strain or misuse the voice often suffer from what is called “clergyman’s sore throat.” Attacks of sore throat due to improper methods of breathing and of using the voice should be treated by judicious elocutionary exercises and a system of vocal gymnastics, under the direction of proper teachers.

Persons subject to throat disease should take special care to wear suitable underclothing, adapted to the changes of the seasons. Frequent baths are excellent tonics to the skin, and serve indirectly to protect one liable to throat ailments from changes in the weather. It is not prudent to muffle the neck in scarfs, furs, and wraps, unless perhaps during an unusual exposure to cold. Such a dress for the neck only makes the parts tender, and increases the liability to a sore throat.

Every teacher of elocution or of vocal music, entrusted with the training of a voice of some value to its possessor, should have a good, practical knowledge of the mechanism of the voice. Good voices are often injured by injudicious management on the part of some incompetent instructor. It is always prudent to cease speaking or singing in public the moment there is any hoarseness or sore throat.

The voice should not be exercised just after a full meal, for a full stomach interferes with the free play of the diaphragm. A sip of water taken at convenient intervals, and held in the mouth for a moment or two, will relieve the dryness of the throat during the use of the voice.

356. Effect of Alcohol upon the Throat and Voice. Alcoholic beverages seriously injure the throat, and consequently the voice, by causing a chronic inflammation of the membrane lining the larynx and the vocal cords. The color is changed from the healthful pink to red, and the natural smooth surface becomes roughened and swollen, and secretes a tough phlegm.

The vocal cords usually suffer from this condition. They are thickened, roughened, and enfeebled, the delicate vibration of the cords is impaired, the clearness and purity of the vocal tones are gone, and instead the voice has become rough and husky. So well known is this result that vocalists, whose fortune is the purity and compass of their tones, are scrupulously careful not to impair these fine qualities by convivial indulgences.

357. Effect of Tobacco upon the Throat and Voice. The effect of tobacco is often specially serious upon the throat, producing a disease well known to physicians as “the smoker’s sore throat.” Still further, it produces inflammation of the larynx, and thus entails disorders of the vocal cords, involving rough voice and harsh tones. For this reason vocalists rarely allow themselves to come under the narcotic influence of tobacco smoke. It is stated that habitual smokers rarely have a normal condition of the throat.