582 



COMPARATIVE ANATOMY. 



as 



various arrangements. For example, there is a muscular process, 

 which is able to partly shut off the cavity which gives off the 



branchial arteries from the rest of the 

 ventricular cavity. The ventricle is com- 

 pletely divided in the Crocodilini. 



The membranous valves of the ostium 

 atrio-ventriculare are greatly developed 

 in the right half of the heart. In the 

 Crocodilini there is only one of these 

 valves on the right side, and it extends 

 along the septum of the ventricle; the 

 other valve is replaced by a process of 

 the lateral muscular wall of this chamber. 

 The arterial bulb, which is single ex- 

 ternally, is apparently given off from 

 the right ventricle. It is, however, 

 divided into a series of canals, which are 

 connected with both ventricles. There 

 are pouched valves at the root of the 

 arteries. 



Of the five primitive arterial arches, 

 the two first have disappeared, and the 

 rest undergo various metamorphoses in 

 the different divisions. In the Saurii 

 the third persists on both sides, and is 



Fig. 329. Heart and arteries 

 of an Ophidian (Boa). 

 d Right, s Left auricle. 

 c Carotid, ad Right, as Left 

 aortic arch. p Pulmonary 

 artery. 



connected on the right with the fourth 



arch, which, like the two branches of 

 the third arch, is given off from the vessel that arises from the 

 left ventricle. The left half of the fourth arch is connected with 

 the third arch on its own side, and so corresponds to the right 

 ventricle. The fifth arch on either side is partly converted into the 

 pulmonary arteries, which primitively spring from it only, and which 

 are given off from the trunk of the pulmonary artery in consequence 

 of the differentation of the primitive aortic bulb. There are, there- 

 fore, two aortic arches on either side, one of which, the second on 

 the left, conveys venous blood. It is, however, connected peri- 

 pherally with the other arches, so that the two kinds of blood must 

 be mixed. In the Ophidii the first pair of arches that persists in 

 the Saurii is not generally connected with . the second pair, but the 

 continuation of this portion is converted into the internal carotid. 

 In the Chelonii the right arterial (Fig. 330, ad), and the left venous 

 aortic arch (as), are connected by a ductus Botalli with the pulmonary 

 arteries, which are developed from the last pair of primitive arteries 

 (pd ps). This disappears in the Crocodilini, so that in them a vessel, 

 which gives off the right aortic arch and the carotids, arises from the 

 left ventricle, while a left aortic arch and the pulmonary arteries 

 arise from the right ventricle. The primitive connection between 

 these vascular trunks is retained in the arterial bulb of the Crocodilini, 

 where the foramen Panizza) is the means of communication between 



