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 



E 1 



FlG. 538. Diagram of mammalian circulation. Hear: : ra, la, right and left auricles; 

 rr, Iv, right and left ventricles;/., capillary system of lungs; K, capillary system of head ; 

 ', E 3 , of anterior and posterior extremities; D, of intestine; P, of liver (portal system). 

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

 subclavian; cm, visceral (coeliac, mesenteric); n, renal; /, iliac; .v, sarnd < c lal). 

 Veins: vp, pulmonary; cs,'ci, pre- and postcavse (precava paired in most vcrtebr.ites); 

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



system remnants may persist, such as a connection (ductus Botalli) of the 

 pulmonary with the aorta (//, <1.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 pulmonate vertebrates systemic blood enters the 

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



