592 



CHORD ATA. 



respiration is far less important than with the Amphibia, lungs, 

 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 cartilage 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 higher groups the whole is chambered, partitions extending 

 inwards to the intrapulmonary 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 systemic and pulmonary 

 blood-vessels occurs (fig. 621). The 

 two auricles are completely separated, 

 while a septum extends into the ven- 

 tricle, complete 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 Panizzae, 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. 621. Heart of crocodile with ar- the pulmonary arteries (4, p). A 



second vessel arises from the right 

 ventricle, is purely arterial and con- 

 nects with most of the remaining 



arterial arches ' the first > which 



vessels from the heart, .and the (aortic arch, ad) of the SCCOnd 

 connexion (foramen Panizzae) be- V ' . ' 



tween the arterial trunk and the arch. The third Vessel Connects Oil 

 left aortic arch, just in front of . . . 



the heart. the one hand with the remaining 



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

 of the heart. The foramen Panizzse occurs between this and the 

 right aortic arch. 



