274 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



Corresponding to the posterior position of the lungs the trachea is 

 long in these groups, but in the anura it can scarcely be said to exist, 

 the lungs succeeding almost immediately to the larynx. 



In the reptiles the trachea varies in length, being shortest in 

 lizards (except amphisbaenas), longer in snakes, tortoises and croco- 

 diles, dividing into bronchi at varying distances from the lungs. 

 It is frequently convoluted in turtles. In many reptiles the cartilage 

 rings of the trachea are incomplete, but in Sphenodon (fig. 301), 

 lizards and some snakes some cartilages (usually the more anterior) 



form complete rings, the others being 

 completed dorsally by membrane. In 

 snakes the successive rings are often 

 united, especially on the sides. 

 y The trachea is greatly elongate in 

 birds in correlation with the length of 

 the neck and the position of the lungs 

 within the thorax. The rings, which 

 are usually complete, are frequently 

 ossified. The trachea is occasionally 

 (male ducks, etc.) widened in the middle 

 and in various groups becomes greatly 

 convoluted so that its length from the 

 glottis to the lungs exceeds that of the 

 neck. In some these convolutions 

 occur beneath the integument of the 

 thorax; in some between the sternum 

 and the muscles; and in the cranes 

 t7t^ .^ c •« c k 1 and swans within the keel of the 



riG. 297, — byrinx of canvas-back 

 duck, Aythya, laid open (Princeton sternum. 

 915). b, bronchi; *, pessulus; t, tra- rr-., i • .^r r 



chea; ly, tympanum. The larynx IS neveT the_orgaivof 



voicp/in__th.e _birdsj its place being 

 taken by a somewhat similar structure, the syriiu^t the division of 

 the trachea into the bronchi. The sound-producing elements are 

 membranes which vibrate by the passage of air, as do the vocal cords 

 of mammals. Most common is the broncho-tracheal syrinx, in which 

 the last rings of the trachea are united to form a resonating chamber, 

 the tympanum, while folds of membrane, internal and external 

 t3rmpaiiic membranes (not to be confused with the similarly named 

 structure in the ear, p. 202), extend into the cavity from the median 

 and lateral wall of each bronchus. In some cases there is also an 



