38 THE ANATOMY OF VERTEBRATED ANIMALS. 



rest of the circulatory system, in the development of tho 

 highest Vertebrata. The primitively simple tube becomes 

 bent upon itself, and divided from before backward into an 

 aortic, or ventricular, and a venous, or auricular, portion. A 

 median septum then grows inward, dividing the auricular and 

 ventricular chambers into two, so that a right auricle and 

 right ventricle become separated from a left auricle and left 

 ventricle. A similar longitudinal division is efifected in the 

 cardiac aorta. The septa are so disposed in the auriculo-ven- 

 tricular chamber that the right auricle communicates with the 

 venous sac and the trunks of the visceral and body veins, 

 while only the veins from the lungs enter into the left auricle. 

 And the cardiac aorta is so divided that the left ventricle com- 

 municates with the chief aortic trunk, the right with the pul- 

 monary artery. Valves are developed at the auriculo-ventric- 

 ular apertures and at the origins of the aortic and pulmonary 

 trunks, and thus the course of the circulation is determined. 

 The septum between the auricles remains incomplete for a 

 much longer period than that between the ventricles — and 

 the aperture by which the auricles communicate is called the 

 foramen ovale. 



In the adult state of Aves and Mammalia, the foramen 

 ovale is closed ; there is no direct communication between the 

 arterial and venous cavities or trunks ; there is only one aortic 

 arch ; and the pulmonary artery alone arises from the right 

 ventricle. In the Orocodilia, the auricles and ventricles of 

 opposite sides are completely separate ; but there are two 

 aortic arches, and one of these, the left, arises from the right 

 ventricle along with the pulmonary artery. In all Reptilia, 

 except Crocodiles, there is but one ventricular cavity, though 

 it may be divided more or less distinctly into a cavum veno- 

 sum and a cavum arteriosum. The auricles are completely 

 separated (except in some Ghelonid), and the blood of the 

 left auricle Hows directly into the cavum arteriosum, while 

 that of the right passes immediately into the cavum venosum. 

 The aortic arches and the pulmonary artery all arise from the 

 cavum, venosum (or a special subdivision of that cavity called 

 the cavum pulmonale) ; the ostium of the pulmonary artery 

 being farthest from, and that of the right aortic arch nearest 

 to, the cavum arteriosum. 



In all Amphibia, the spongy interior of the ventricle is 

 andivided, and the heart is trilooular, though the auricular 

 septum is sometimes small and incomplete. In all Pisces, ex- 

 cept Lepidosiren, there is no auricular septum. In Aniplil- 



