IV. VERTEBRATA. 



485 



the heart, occurs only in birds and mammals. Reptiles and 

 amphibia show how the modification has been accomplished. In 

 these the separation begins in the venous system and extends to the 

 auricle, in the reptiles the septum arises in the ventricle. In the arterial 



Fig. 538. — Diagram of mammalian circulation. Heart: ra, la, right and left auricles; 

 rv, h\ riglit and left ventricles ;i, capillary system of lungs; K, capillary system of head ; 

 E', E-, of anterior and posterior extremities; D, o£ intestine; P, of liver (portal system). 

 Arteries: ap, pulmonary artery; a, ascending aorta; ad, descending aorta; c, carotid; s, 

 subclavian; cm, insceral (coeliac, mesenteric); n, renal; /, iliac; s, sacral (caudal). 

 Veins: vp, pulmonar)'; cs, ci, pre- and postcavn; (precava paired in most vertebrates); 

 i, jugular; s, subclavian; vh, hepatic; p, portal; n, renal; i, iliac; s, sacral. 



system remnants may persist, such as a connection {ductus BotaUi) of the 

 pulmonary with the aorta (//, d.B), or one aortic arch may arise with 

 the pulmonalis from the right side of the heart (///, ao). 



As a result of this in the pulmonale vertebrates systemic blood enters the 

 right auricle by the pre- and postcavcc, then passes through the right ventricle 



