Taking the thyroid cartilage as the fixed base, the cricoid and arytenoid cartilages undergo movements which bring about two distinct sets of changes in the glottis and its elastic edges, namely, (1) widening and narrowing the opening; (2) stretching and relaxing of the vocal cords. During ordinary respiration the glottis remains about half open, being slightly widened during inspiration (b'). During forced inspiration the glottis is widely dilated by muscular action (c'). If an irritating gas be inspired, the glottis is tightly closed by a spasmodic action of certain muscles, so that the true vocal cords act as a kind of valve.

During vocalization the glottis is formed into a narrow chink with parallel sides (a'), while the cords are made more or less tense, according to the pitch of the note to be produced; both these changes are brought about by muscular action.

The opening of the chink of the glottis is accomplished chiefly by a muscle called the posterior crico-arytenoid, which passes from the posterior surface of the cricoid cartilage to the outer and posterior angle of the arytenoids. By pulling the latter point downward and backward it separates the arytenoid cartilages, particularly at their anterior extremity, where the cords are attached. In this action it is aided by a small muscle connecting the posterior surfaces of the arytenoid, namely, the posterior arytenoid, which tends, when the two arytenoid cartilages are held apart, to rotate them, so that the vocal processes are separated.

Diagrams taken from the laryngoscopic view of the larynx.

Fig. 197. Diagrams taken from the laryngoscopic view of the larynx, showing in transverse section the position in which the vocal cords and the arytenoid cartilages are supposed to be during different actions of the larynx.

A'. Vocal chink, as in singing.

B'. In easy, quiet inhalation of air.

C. In forced inspiration.

Diagram of the side view of the larynx, showing the position of the vocal cords (V).

Fig. 198. Diagram of the side view of the larynx, showing the position of the vocal cords (V). {Huxley.) Ar. Arytenoid cartilage. Hy. Hyoid bone. Th. Thyroid cartilage. Cr. Cricoid cartilage. Tr. Trachea.

C. th. Crico-thyroid muscle. Th. A. Thyro-arytenoid muscle. Ep. Epiglottis.

Diagram of the opening of the larynx from above.

Fig.199. Diagram of the opening of the larynx from above. {Huxley.) Th. Thyroid cartilage. Cr. Cricoid cartilage. Ary. Superior extremities of the arytenoid cartilages. V. Vocal cords.

Th. A. Thyro-arytenoid muscles. C.'a. I. Lateral crico-arytenoid muscle. C. a. p. Posterior crico-arytenoid muscle. A.r.p. Posterior arytenoid muscle.

The narrowing of the glottis is executed by the lateral cricoarytenoids which run upward and backward from the anterolateral aspect of the cricoid to the muscular processes of the arytenoid cartilages.' They pull the muscular processes forward, and thus rotate the arytenoid cartilages so as to approximate the vocal processes to one another, while any tendency toward pulling apart the bodies of the cartilages, owing to the downward direction of the muscle, is overcome by the posterior arytenoid muscle and those muscular bands which pass from the posterior surface of the arytenoid cartilages to the epiglottis and the upper part of the thyroid cartilage, the external thyro-arytenoid, and the thyro-ary-epiglottic muscles (Henle). The other fibres, which pass directly from the arytenoid to the thyroid cartilages - internal and external thyro-arytenoid muscles - in the same direction as the vocal cords, complete the closure by helping to press together the vocal processes, and by approximating the cords themselves. In spasmodic closure of the glottis, all these latter muscles act violently together, and have been grouped by Henle as the constrictor of the glottis.

Relaxation of the vocal cords accompanies voluntary closure of the glottis, as in holding the breath, when the false vocal cords are said to have a valvular action. The muscular fibres which run from the arytenoid cartilages to the thyroid, nearly parallel to the true vocal cords, are those concerned in the act of relaxation when the cords are active. They pull forward the arytenoid cartilages, and at the same time draw the upper part of the cricoid slightly forward. These muscles have the all-important action of adapting the edges of the cords and the neighboring surfaces to the exact shape most advantageous to their vibration.

The tightening of the vocal cords is caused by a single muscle, the crico-thyroid, which, on the outer side of the larynx, passes downward and forward from the lower part of the thyroid to the anterior part of the cricoid cartilage. It pulls the anterior part of the cricoid cartilage upward, causing it to rotate round an axis passing through its thyroid joints. The upper part of the cricoid, which carries the arytenoids, moves backward, the attachments of the vocal cords are separated, and the membranes are thus put on the stretch.

The requirements necessary for the production of voice are the following: -

1. Elasticity of the vocal cords and smoothness of their edges; freedom from all surface irregularity, such as would be caused by thick mucus adhering to them, or by any abnormality.

2. The cords must be very accurately adjusted, and closely approximated together, so that they almost touch evenly throughout their entire length.

3. The cords must be held in a certain degree of tension, or their vibration cannot produce any vocal tone, but only a raucous noise.

4. The air must be propelled through the glottis by a forced expiration. The normal expiratory current is too gentle to give the necessary vibration. After the operation of tracheotomy, the air escapes through the abnormal opening, and sufficient pressure cannot be brought to bear on the cords, so no vocal sound can be produced, and the person speaks in a whisper, unless the exit of air through the tracheotomy tube is prevented by placing the finger temporarily upon the opening.