SUMMARY.
The larynx is the most important organ in voice-production, and consists of cartilages, muscles, the vocal bands, true and false, membranes and ligaments, folds of mucous membrane, etc. It is situated between the hyoid (tongue) bone above and the trachea below. The cartilages are the (1) epiglottis, (2) thyroid, cricoid, arytenoid, the two small, unimportant cornicula laryngis, or cartilages of Santorini, surmounting the arytenoids, and the two cuneiform, or cartilages of Wrisberg, in the folds of mucous membrane on each side of the arytenoids.
The muscles are attached to the main cartilages. In addition to the muscles that are concerned with the movements of the vocal bands, others that hold the larynx in place or raise and lower it are attached externally to these, especially to the large thyroid cartilage. The epiglottis, the false vocal cords, the true vocal cords, and the thyro-arytenoid muscles are attached to the interior anterior surface of the thyroid in this order from above down.
The false vocal bands have no direct function in phonation. The whole larynx, so far as phonation is concerned, may be said to exist for the true vocal bands. They are attached close together to the internal and anterior surface of the thyroid in front and to the lower anterior angles (vocal processes) of the arytenoids behind. Between the false vocal bands above and the true vocal bands below there is a cavity (the ventricle of Morgagni). The false vocal bands are protective, and approximate closely during coughing, swallowing, etc.
It is very important to note that the arytenoid cartilages move freely on their base, swivel-like, so that nearly all the changes effected in the movements and tension of the vocal bands are brought about through alterations in the position of these cartilages; and this implies that all the muscles concerned are attached to them. From above down, in front, the order of structures is as follows:
Hyoid bone.
Membrane.
Thyroid cartilage.
Membrane.
Cricoid cartilage.
Trachea.
The hyoid bone is not a part of the larynx, but from it the larynx is suspended. The bone itself gives attachment to the muscles of the tongue. The glottis is the chink between the true vocal bands.
The muscles of the larynx may be divided into the following: (1) Those that open and those that close the glottis; (2) those which regulate the tension of the vocal bands. The latter include the (a) crico-thyroids, which tense and elongate them, (b) thyro-arytenoids, which relax and shorten them. The crico-thyroid may be considered the most important muscle of phonation, because it is so much used and so effective. By its action the cricoid is pulled up in front and down behind, so that the arytenoids are drawn back, and thus the vocal bands tensed and lengthened. The lateral crico-arytenoids and the thyro-arytenoids have the opposite effect—i.e., they relax and shorten the vocal bands; hence when they come into play a new register begins. The thyro-arytenoids, attached along the whole length of the vocal bands externally, have a very important but not well-understood action in the production of the higher tones, and probably also of the falsetto.
The whole larynx is lined with mucous membrane, that covering the true vocal bands being very thin. The false vocal bands are made up chiefly of mucous membrane; the true vocal bands abound in elastic tissue. The larynx rises during the production of high tones, and during phonation its vibrations may be felt, as also those of the chest.