166 REPTILIA. 



them, as well as in the single ventricle. In the Tailed Batrachiathe 

 aortic arches unite at an early period of existence and high up ; in 

 the Anourous kind very low down to form the abdominal aorta. 



In the Squamigerous Reptiles, as in the Chelonia, the heart is 

 larger and stronger, the auricles separated externally and provided 

 with a stronger muscular partition, and the ventricle is divided into 

 two cavities, by a more or less perforate or complete septum, traces 

 of which occur, in Pipa, among the Batrachia. The ventricle is 

 properly the right one of the higher animals developed to a greater 

 degree, and the conus arteriosus is in like manner a special division 

 of the cavity from which the aorta and pulmonary artery arise. 

 Between the auricles and ventricles are interposed strong valves, 

 and the usual valv. semilunares at the commencement of the 

 arteries. Each of the two main branches of the pulmonary artery 

 gives off a large ductus arteriosus to join one of the two aortic arches, 

 which pass over the corresponding branch of the trachea to the 

 vertebral column, in order to unite and form the trunk of the abdom- 

 inal aorta. 



Manifold diversities occur in the circulatory system of the several 

 orders of Squamigerous Reptiles ; thus the heart of the Ophidia, and 

 also of the Sauria, is more elongated, while on the contrary in the 

 Chelonia it is short and very broad. The true Ophidia on account 

 of their single lung have only the left branch of the pulmonary ar- 

 tery developed, which gives off its ductus arteriosus, and a twig to 

 the rudiment of the right lung. The two trunks of the pulmonary 

 veins usually enter the left auricle, united together, but rarely sep- 

 arate. 



The Crocodiles exhibit the most complete form of heart, for in 

 them the arrangement of its several parts agrees essentially with 

 that of Birds and Mammalia. The walls of the heart are very thick 

 and muscular, and the two ventricles are completely separated by a 

 strong septum from each other. But just at the outlet of the ven- 

 tricles we find a communication established between the two, so 

 that the two kinds of blood, to wit, the vitiated and purified, become 

 mixed together, and thus a similarity is established with the rest of 

 the Reptilia, and the foetal condition of the heart in Birds and Mam- 

 malia. The ductus arteriosus of Botal is to a certain extent persis- 

 tent, and the aorta arises by an externally single, but internally double 

 trunk from the right as well as left ventricle of the heart, and thus 

 forms two corresponding aortae. 



in all Reptiles the heart is surrounded by a pericardium, which 



