RESPIRATORY SYSTEM, ANATOMY OF. The respiratory tract consists of the nasal cavities, pharynx, larynx, trachea, bronchi and lungs. For the first two parts see OLFACTORY SYSTEM and PHARYNX.
The thyroid cartilage is the largest in the larynx and consists of two plates or aloe joined in the mid-ventral line. At the upper part of their junction is the thyroid notch and just below that is a forward projection, the pomum Adami ("Adam's apple"), best marked in adult males. From the upper part of the posterior border of each ala the superior cornu rises up to be joined to the tip of the great cornu of the hyoid bone by the lateral thyro hyoid ligament, while from the lower part of the same border the inferior cornu passes down to be fastened to the cricoid cartilage by the crico-thyroid capsule. From the upper border of each ala the thyro-hyoid membrane runs up to the hyoid bone, while near the back of the outer surface of each the oblique line of the thyroid cartilage runs downward and forward.
The cricoid cartilage (see figs. I and 2) is something like a signet ring with the seal behind; its lower border, however, is horizontal. To the mid-ventral part of its upper border is attached the mesial part of the crico-thyroid membrane, which attaches it to the lower border of the thyroid cartilage; the lateral parts of this mem brane pass up internally to the thyroid cartilage and their upper free edges form the true vocal cords. On the summit of the signet part of the cricoid are placed the two arytenoid cartilages (see fig. 2), each of which forms a pyramid with its apex upward. The base articulates with the cricoid by a concave facet, sur rounded by the crico-arytenoid capsule, and the two arytenoids can glide toward or away from one another, while each can rotate round a vertical axis. From the front of the base a delicate
process projects which is attached to the true vocal cord (vocal process), while from the outer part of the base a stouter process attaches the two crico-arytenoid muscles (muscular process).
cricoid, by pulling up which they make the upper part of the signet, with the arytenoids attached to it, move back and so tighten the vocal cords. (2) The t/iyro-arytenoids (see fig. 4), which run back from the junction of the thyroid alae to the front of the arytenoids and side of the epiglottis; they pull the aryte noids toward the thyroid and so relax the cords. (3) The single The epiglottis (see fig. 3) forms a lid to the larynx in swallow ing; only the box moves up to the lid instead of the lid moving down to the box. It is leaf-shaped, the stalk being attached to the junction of the thyroid cartilages inside the larynx, while the anterior surface of the leaf is closely attached to the root of the tongue and body of the hyoid bone. The posterior Or laryngeal surface is pitted for glands. All the cartilages of the larynx are of the hyaline variety except the epiglottis, the cornicula laryngis and the cuneiform cartilages, which are yellow elastic. The result is that all except these three tend to ossify as middle age is approached.
The muscles of the larynx are : (I) the crico-thyroids, attached to the lower border of the thyroid and the anterior part of the arytenoideus muscle, which runs from the back of one arytenoid to the other and approximates these cartilages. (4) The lateral crico-arytenoids (see fig. 4) which draw the muscular processes of the arytenoids forward toward the ring of the cricoid and, by so doing, twist the vocal processes, with the cords attached, inward toward one another; and (5) the posterior crico-arytenoids (see fig. 4) which run from the back of the signet part of the cricoid to the back of the muscular processes of the arytenoid and, by pulling these backward, twist the vocal processes outward and so separate the vocal cords. All these muscles are supplied by the recurrent laryngeal nerve, except the crico-thyroid which is inner vated by the external branch of the superior laryngeal (see NERVE, Cranial).