254 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In the whales and young marsupials the larynx is prolonged so that it projects 

 into the choana behind the soft palate. In the whales (fig. 262) this is an adaptation 

 to the manner of taking food from the water and breathing at the same time. In 

 the young marsupials the milk is forced into the mouth by the muscles of the mam- 

 mae of the mother and this arrangement prevents strangulation. 



Trachea. In the tetrapoda the trachea is strengthened by the 

 formation of cartilage in its walls, the beginnings of which are seen 

 in the urodeles where the fifth branchial arch gives rise to these ele- 

 ments (p. 251). Their arrangement varies considerably in the urodeles 

 and csecilians, being sometimes scattered pieces, 

 sometimes regularly arranged and even united in 

 the lateral walls (fig. 258). 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 crocodiles, divid- 

 FIG. 262. Larynx of ing into bronchi at varying distances from the 

 Xiphius cavirostris (after lungs> It is frequently bent in turtles. In many 



Gegenbaur) from side . J 



showing the prolongation reptiles the cartilage rings of the trachea are in- 

 complete, but in Sphenodon, lizards and some 



into the choana ; c, cricoid ; snakes some cartilages (usually the more anter- 



th, thyreoid. 



ior) form complete rings, the others being com- 

 pleted dorsally by membrane. In snakes the successive rings are 

 often united, especially on the sides. 



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 convolu- 

 tions occur beneath the integument of the thorax; in some between the 

 sternum and the muscles; and in the cranes and swans within the 

 keel of the sternum. 



The larynx is never the organ of voice in the birds, its place being 

 taken by a somewhat similar structure, the syrinx, at the division of 

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



