A Practical Physiology Part 38

Experiment 184. Look at the row of letters (S) and figures (8). To

[Ill.u.s.tration: S S S S S S S S 8 8 8 8 8 8 8 8 ]

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.

Experiment 185. _To ill.u.s.trate 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.

[Ill.u.s.tration: Fig. 147.--To show False Estimate of Size.

/ / /| | / | | | | A | B | | | | / | |/ | / / ]

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 sophagus on its pa.s.sage 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 pa.s.s from the brain to the parts below.

We have already described the food pa.s.sages (Chapter VI.) and the air pa.s.sages (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 pa.s.sages, 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 pa.s.sage,--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 ma.s.s of mucus, which irritates the pa.s.sages 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 pa.s.sages.

[Ill.u.s.tration: 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.)

A, hyoid bone; B, thyro-hyoid membrane; C, thyroid cartilage; D, erico-thyroid membrane; E, cricoid cartilage, lateral ligaments seen on each side; F, upper ring of the trachea.

("Adam's apple" is in the V-shaped groove on a line with B and C.) ]

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.

[Ill.u.s.tration: Fig. 149.--Diagram of a Sectional of Nasal and Throat Pa.s.sages.

C, nasal cavities; T, tongue; L, lower jaw; M, mouth; U, uvula; E, epiglottis; G, larynx; O, sophagus.

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, pa.s.ses 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 c.h.i.n.k the s.p.a.ce between the part of the larynx above them and the part below them. This constriction of the larynx is called the glottis.

[Ill.u.s.tration: Fig. 150.--View of the Cartilages and Ligaments of Larynx.

(Posterior view.)

A, epiglottis; B, thyroid cartilage; C, arytenoid cartilage; D, ligament connecting lower cornu of the thyroid with the back of the cricoid cartilage; E, cricoid cartilage; F, upper ring of the trachea.

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 s.p.a.ce 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.

[Ill.u.s.tration: Fig. 151.--Longitudinal Section of the Larynx. (Showing the vocal cords.)

A, epiglottis; B, section of hyoid bone; C, superior vocal cord; D, ventricle of the larynx; E, inferior vocal cord; F, section of the thyroid cartilage; H, section of anterior portion of the cricoid cartilage; K, trachea; L, section of the posterior portion of the cricoid cartilage; M, arytenoid cartilage; N, section of the arytenoid muscle.

353. Factors in the Production of the Voice. Muscles which pa.s.s 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 s.p.a.ce 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 ba.s.ses 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 pa.s.sages, 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.

[Ill.u.s.tration: 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.