XI RESPIRATORY ORGANS 517 



Organs of Respiration and Voice. Owing to the 

 presence of a neck, the lungs are situated at 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 

 just ventrally to the gullet, its anterior end forming 

 the larynx or organ of voice (p. 506), and communi- 

 cating with the pharynx through the glottis. The 

 cartilaginous rings of the trachea are incomplete dor- 

 sally, 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 



