xi RESPIRATORY ORGANS 517 



Organs of respiration and voice, Owing to the presence 

 of a neck, the lungs are situated some distance from the 

 glottis (Fig. 135), and, instead of a short laryngo-tracheal 

 chamber, as in the frog (p. 141), there is a windpipe or 

 trachea (tr) extending along the neck (p. 506) just ventrally 

 to the gullet, its anterior end forming the larynx or organ of 

 voice, and communicating with the pharynx through the 

 glottis. The cartilaginous rings of the trachea are incom- 

 plete dorsally, and the cartilages of the larynx are more 

 highly differentiated than in the frog, apart from the presence 

 of an epiglottis (epg, see p. 512). The largest and most anterior 

 laryngeal cartilage is the thyroid, which, like the epiglottis, 

 is peculiar to Mammals : it has the form of a broad ring, 

 incomplete dorsally, and is the part of the larynx which can 

 be felt externally. The second cartilage is the cricoid, 

 represented in the frog by a ring-shaped cartilage at the base 

 of the lungs (p. 153) : its form is somewhat like that of a 

 signet ring, being broad dorsally where it lies mainly 

 between the edges of the thyroid and narrow ventrally. 

 A pair of arytenoid cartilages are articulated to the dorsal 

 and inner surface of the cricoid, and each is produced 

 into a projecting process situated between the two edges of 

 the thyroid cartilage. The vocal cords (p. 144) are a pair 

 of elastic folds extending across the cavity of the larynx 

 from the thyroid below to the arytenoids above, each 

 bounded in front by a depression. 



In the position of rest, the vocal cords lie at an acute angle to one 

 another, as in the frog ; they can be brought into parallelism and 

 regulated by the action of a number of intrinsic and extrinsic muscles, 

 and are set in vibration by the respiratory current of air. 



After entering the thorax, the trachea divides into two 

 bronchi (Fig. 135, br), one entering each lung and giving off 

 branches to its different lobes : the bronchi, like the trachea, 



