Before describing the muscles of the larynx, the reader is reminded of the order of structures from above downward, in front, which is as follows:
The hyoid bone.
The thyro-hyoid membrane.
The thyroid cartilage.
The crico-thyroid membrane.
The cricoid cartilage.
The trachea.
The latter is connected with the cricoid cartilage by its membrane.
All the above structures can be felt in one's own person, the more readily if he be thin and have a long neck. The hyoid bone, or tongue-bone, is that hard structure just above the cricoid cartilage, and which one may easily demonstrate to be much more movable than the larynx itself. The tongue muscles are attached to it above, and from it, below, the larynx is suspended, as already explained.
The muscles of the larynx are best understood if the principle of antagonistic action already referred to be remembered. Speaking generally, the muscles are arranged in pairs which have an opposite or antagonistic action—viz.: (1) Those that open and close the glottis; (2) those that regulate the tension, or degree of tightness, of the vocal bands.
1. The muscles whose action tends to approximate the vocal bands—the adductors—are the arytenoidēus proprius and the thyro-arytenoidēus. The former is attached to the posterior or back surface of both arytenoid cartilages; the latter, as its name indicates, to the anterior and inner surface of the thyroid and the anterior lower surface or angle (vocal process) of the arytenoid.
Fig. 30 (Chapman). Diagram showing action of crico-thyroid muscle, stretching of the vocal cords, and lengthening of them. The dotted lines indicate the position assumed when the muscle has contracted.
The opening or widening of the glottis is effected on each side (one muscle of the pair and its action being alone described in this and other cases) by the antagonist of these muscles, the crico-arytenoidēus posticus, whose attachments are exactly as indicated by the names—viz., to the posterior part of the two cartilages named. When reading the description of these or other muscles it is absolutely necessary to have a pictorial illustration or the real object before one. The pull of this muscle is from the more fixed point, as in all other cases; hence the force is applied in a direction from below and outward, with the result that the arytenoid cartilage is tilted outward, and with it the vocal band is moved from the middle line.
