592 



CHORDA TA. 



respiration is far less importuiit than with the Amphibia, Inngs, 

 as in birds and mammals, being the respiratory organs, and in 

 these a progressive development may be followed. The larynx is 

 followed by a trachea with cai-tilage supports in its wall, and this 

 either opens directly into the two lungs or divides into two bronchi, 

 which, in Varanus, may divide again inside the lungs. The lungs 

 in the more primitive forms are subdivided only peripherally, but 

 in the higlier groups the whole is chambered, partitions extending 

 inwards to the intrapulmouary bronchus. 



Since the respiration is entirely pulmonary, the heart is divided 



into a left arterial and a right 

 venous half, and a corresponding 

 separation of sj'stemic and pulmonary 

 blood-vessels occurs (fig. 021). The 

 two auricles are completely separated, 

 while a septum extends into the ven- 

 tricle, comjilete in the crocodiles, 

 but not in turtles, lizards, and snakes. 

 Yet even in the crocodiles a mixing 

 of arterial and venous blood occurs 

 since in the large aortic trunks which 

 arise from both ventricles a commu- 

 nication, the foramen Panizza?, per- 

 sists. The arterial trunk is divided 

 by internal partitions into three ves- 

 sels, which are but rarely visible from 

 the exterior. One of these arises 

 from the right ventricle, carries 

 venous blood, and takes over the 

 fourth arterial arch, which gives off 



Fig. 62i.-Heart of crocodile with ar- the pulmonary arteries (4, /))• -^ 

 teries (diat^ram). a' a^, riglit and i i ■ p ,i • i x 



left auricles; ad, as, right and left SCCOud VCSSel uriSCS from the right 



aortic arches; c,. carotids; ,,.,0=, ,.,.j,tricle, is purely arterial and con- 

 nects with most of the remaining 

 arterial arches, the first, which gives 



n!^?''i';->,^""'i&''^K''^t,'^"'' «io>ie of ^,ff tlie carotid, and the rioht half 



amphibia. Notice the ongm of the ^ ^ 



arch, ad) of the second 

 The third vessel connects on 

 hand with the remaining 

 (left, second) arch and on the other with the right or venous half 

 of the lieart. The foramen Panizzas occurs between this and the 

 riirht aortic arch. 



right and left auriculo-ventricnlar 

 ostia; p, pulmonary artery; .s, sub- 

 clavians; u^ v'^^ right and left ven- 

 tricles f the arrows show the direc 

 tion of the blood flow); J, 2, U 

 7\W\ tho 

 . . 3 origin ( _ _ 

 vessels from the heart, and the (;^ortic 

 connexion (foramen Panizza') be- ^' 

 tween the arterial trunk and the j^'ch. 

 left aortic arch, just in front of 

 the heart. the OUe 



