THE ORGANS OF RESPIRATION. 



THIS tract includes the organs by which an interchange of gases takes place 

 between the blood and the air. It consists of the larynx, the trachea or windpipe, 

 and its subdivisions, the bronchi, the lungs, and the serous membranes, \\\epleur<z, 

 which surround them. Morphologically this tract is an outgrowth from the fore- 

 gut. The larynx is a specialized apparatus for the production of the voice, situated 

 at the beginning of the windpipe, of sufficient importance to be considered by itself. 



THE LARYNX. 



The larynx consists of a number of cartilages which, by their relative changes 

 of position, modify the approximation and tension of two folds of mucous mem- 

 brane over fibrous tissue, known as the vocal cords, on either side of the cleft through 

 which the air enters the windpipe. The larynx is in the neck, being suspended from 

 the hyoid bone and leading to the trachea. It is practically subcutaneous in front. Its 

 superior orifice is behind the base of the tongue, and can be seen in life only by a mirror. 

 The cartilages are connected by joints and ligaments, moved by muscles, and covered 

 by mucous membrane, the folds of which form important morphological parts of the 

 larynx. 



THE CARTILAGES, JOINTS, AND LIGAMENTS. 



The cartilages which form the framework of the larynx are three single ones : the 

 cricoid, && thyroid, and the epiglottis; and three pairs : \\i& arytenoid cartilages, the cor- 

 nicula laryngis or cartilages of Santorini, and the cuneiform cartilages or those of 

 Wrisberg. The last pair, although determining well-defined swellings of the mucous 

 membrane, are very small ; indeed, the cartilage is not always to be found. There are 

 other minute points of cartilage to be mentioned with the structures in which they occur. 



The epiglottis, the upper part of the cartilages of Santorini, those of Wrisberg, and 

 the ends of the vocal and apical processes of the arytenoids consist of elastic cartilage, the 

 others being of hyaline cartilage. The cricoid and arytenoid cartilages are derivations 

 from the trachea and represent the more primitive form of larynx. The thyroid and the 

 epiglottis appear in mammals. In monotremes the epiglottis is of hyaline cartilage. 



The Cricoid Cartilage. This is the foundation of the larynx, being a ring 

 on the top of the trachea. It is nearly circular, the diameter in the male being 19 

 mm. (Luschka). It is narrow in front, being from 3-8 mm., usually about 5 mm. 

 broad, and some four or five times as much 



behind. The height at the back is approxi- FIG. 1538. 



mately 25 mm. in the male and from 16-23 

 mm. in the female. The cricoid is 3 or 4 mm. Articu1ar facet 

 thick in the lower part and in the upper as for arytenoid 

 much as 5 or 6 mm. The posterior aspect is 

 somewhat quadrilateral, the upper border de- 



i- i , ,1 i T , i] Articular facet 



scenchng very steeply at the sides. Internally for thyroid 

 the cricoid is perfectly smooth. The lower cartilage 

 border presents a slight median descent in front Cricoid cartilage, right lateral aspect. 



and an inconspicuous notch behind. Never- 

 theless, the cricoid is so placed that its posterior margin is a trifle the lower. A small 

 median depression occurs in the superior border behind, and on either side is an 

 articular eminence for the arytenoid cartilage. Being situated on the superior border 

 of the cricoid, this elongated eminence has its long diameter (8-10 mm.) slanting 

 outward, downward, and somewhat forward. Its free edge may be slightly convex 

 or concave in the long axis, but is not far from straight. It is convex transversely 

 and about 4 mm. thick. The whole elevation is inclined slightly away from the 

 interior of the larynx, so as somewhat to overhang its posterior surface, and is 



1813 



