OF THE SPECIAL ACTIVITIES AND INSTINCTS.

Under this head would fall to be considered Locomotion, the Voice, Mastication, and the Constructive and Destructive apparatus, &c. Locomotion is an interesting subject in itself, but for the purposes of the present treatise I do not consider the full exposition of it at all necessary. The notice already taken of the subject in the earlier part of this chapter must

suffice.

33. So deeply does the act of Speech enter into the operations of Mind-Emotion, Action, and Intelligence-that the mechanism of the organ deserves a full description in this place.

Of the Voice.

I shall first make a few quotations from the Anatomy of the Voice.

The upper part of the air passage (from the lungs) is modified in its structure to form the organ of voice. This organ, named the larynx, is placed at the upper and fore part of the neck, where it forms a considerable prominence in the middle line. It lies between the large vessels of the neck, and below the tongue and hyoid bone, to which bone it is suspended.'

'The larynx is cylindrical at the lower part, where it joins the trachea (or windpipe), but it widens above, becomes flattened behind and at the sides, and presents a blunted vertical ridge in front.

'The larynx consists of a framework of cartilages, articulated together and connected by proper ligaments, two of which, named the true vocal cords, are immediately concerned in the production of the voice. It also possesses muscles, which move the cartilages one upon another, a mucous membrane lining its internal surface, numerous mucous glands, and lastly, blood-vessels, lymphatics, and nerves, besides cellular tissue and fat.'

Cartilages of the Larynx. The cartilages of the larynx

consist of three single and symmetrical pieces, named respectively the thyroid cartilage, the cricoid cartilage, and the cartilage of the epiglottis, and of six others, which occur in pairs, namely, the two arytenoid cartilages, the cornicula laryngis, and the cuneiform cartilages. Of these, only the thyroid and cricoid cartilages are seen on the front and sides of the larynx (see fig. p. 302); the arytenoid cartilages, surmounted by the cornicula of the larynx, together with the back of the cricoid cartilage, on which they rest, form the posterior wall of the larynx, whilst the epiglottis is situated in front.'-QUAIN, p. 1159.

Confining ourselves as much as possible to the parts immediately connected with voice, I require to call attention principally to the thyroid and cricoid cartilages, the two arytenoid cartilages, the true vocal cords, and the muscles that move the cartilages and thereby affect the tension and position of the vocal cords.

"The thyroid (shield-shaped) cartilage (see fig. 10) is the largest of the pieces composing the larynx. It is formed by two flat lamellæ, united in front at an acute angle along the middle line, where they form a vertical projection which becomes gradually effaced as it is traced from above downwards. The two lamellæ, diverging one from the other backwards, embrace the cricoid cartilage, and terminate posteriorly by two thick projecting vertical borders, separated widely from each other; hence the thyroid cartilage is altogether wanting behind. The angular projection on the anterior surface in the median line is subcutaneous, and is much more prominent in the male than in the female, being named in the former the pomum Adami.'

'The cricoid cartilage, so named from its being shaped like a ring, is thicker in substance and stronger than the thyroid cartilage; it forms the inferior, and a considerable portion of the back part of the larynx, and is the only one of the cartilages which completely surrounds this organ. It is deeper behind, where the thyroid cartilage is deficient, measuring in the male about an inch from above downwards, but is much narrower in front, where its vertical measurement is

only two lines and a half. The cricoid cartilage is circular below, but higher up it is somewhat compressed laterally, so that the passage through it is elliptical, its antero-posterior diameter being longer than the transverse.'

'The arytenoid (ewer-shaped) cartilages (fig. 11) are two in number, are perfectly symmetrical in form. They may be compared to two three-sided pyramids recurved at the summit, measuring from five to six lines (half an inch) in height, resting by their bases on the posterior and highest part of the cricoid cartilage, and approaching near to one another towards the median line. Each measures upwards of three lines in width, and more than a line from before backwards.'-p. 1162.

The cartilages are bound together by ligaments, of which I omit the description. The appearance of the interior of the larynx is given as follows (see fig. 11):

'On looking down through the superior opening of the larynx (where it communicates with the pharynx above and is bounded by the epiglottis, &c.), the air passage below this part is seen to become gradually contracted, especially in its transverse diameter, so as to assume the form of a long narrow fissure running from before backwards. This narrow part of the larynx is called the glottis. Below it, at the upper border of the cricoid cartilage, the interior of the larynx assumes an elliptical form, and lower down still it becomes circular. The glottis is bounded laterally by four strongly marked folds of the mucous membrane, stretched from before backwards, two on each side, and named the vocal cords. The superior vocal cords are much thinner and weaker than the inferior, and are arched or semi-lunar in form; the inferior or true vocal cords are thick, strong, and straight. Between the right and left inferior vocal cord is the narrow opening of the glottis, named the rima glottidis, and sometimes the glottis vera, or true glottis.'-p. 1167.

The inferior or true vocal cords, by whose vibration the voice is produced, are two bands of elastic substance, attached in front to about the middle of the depression between the wings of the thyroid cartilage, and behind to the arytenoid cartilages; from this connexion they are called thyro-arytenoid

ligaments. They consist of closely arranged parallel fibres, of that peculiar tissue occurring in some other parts of the body, named the yellow elastic tissue, being probably the most perfectly elastic substance of a ligamentous kind that nature has produced. India-rubber is employed, as an extremely inferior imitation, in making artificial instruments resembling the larynx. The upper and free edges of the cords, which are sharp and straight, are the parts thrown into vibration during the production of the voice.

34 With reference to the muscles of the larynx I may state beforehand that the principal movements to be effected by them relate to the change of tightness and change of distance of the two cords, for which purposes opposing pairs are necessary. By one action the cords are tightened, by another

FIG. 10.*

relaxed; by a separate action they are approximated, and by the antagonist of this they are parted asunder.

The great muscle of tension of the cords, the foremost and most powerful of all the muscles of the voice, is the crico-thyroid exhibited in the figure. It is a short, thick triangular muscle, seen on the front of the larynx, situated on the fore part and side of the cricoid cartilage. It arises by a broad origin from the cricoid cartilage, reaching from the median line backwards upon the lateral surface, and its fibres, passing obliquely upwards and outwards, and diverging slightly, are inserted into the lower border of the thyroid cartilage.' The

*Side view of the thyroid and cricoid cartilages, with part of the trachea; after Willis.-8. Thyroid cartilage. 9. 9. Cricoid cartilage. 10. Crico-thyroid muscle. II. Crico-thyroid membrane, or ligament. 12. Upper rings of the trachea.'-(QUAIN, p. 1171.)

THE LARYNX AS AN INSTRUMENT OF SOUND. 303

contraction of the two crico-thyroid muscles causes the thyroid and cricoid cartilages to turn on each other behind; thus if we suppose the cricoid cartilage

to remain fixed, the upper part of the thyroid is carried forward or away from the other, drawing with it the ends of the vocal cords, which are attached behind to the cricoid cartilages through the arytenoid (see fig. 11). In this way the vocal cords are stretched in proportion as the muscle contracts itself. The counteracting or antagonistic muscles are exhibited in the fig. (No. 7)

FIG. 11.*

passing between each arytenoid cartilage and the thyroid near the extremity of the vocal cords.

In governing the aperture of the glottis, we find a muscle passing between the two arytenoid cartilages (6), and therefore by its contraction drawing them together, and thus approximating the cords. The cords are separated and the glottis widened by a pair of muscles exhibited in the figure (4, 4) passing between the arytenoid and cricoid cartilages behind. No. 5 in the figure is another muscle connecting the same two cartilages laterally, and operating to contract the glottis.

35. The Larynx, considered as an instrument for the production of sound.-It has long been a question what kind of instrument the larynx should be compared to, in order to illustrate the manner of its action in giving out sound. From the existence of two vibrating strings or cords, the first

A diagram, slightly altered from Willis, showing a bird's-eye view of the interior of the larynx.-1. Opening of the glottis. 2. 2. Arytenoid cartilages. 3. 3. Vocal cords. 4.4 Posterior crico-arytenoid muscles. 5. Right lateral crico-arytenoid muscle; that of the left side is removed. 6. Arytenoid muscle. 7. Thyro-arytenoid muscle of the left side; that of the right side is removed. 8. Upper border of the thyroid cartilage. 9. 9. Upper border and back of the cricoid cartilage. 13. Posterior cricoarytenoid ligament.'-(QUAIN, p. 1172.)