CIRCULATORY ORGANS 



325 



arise. In their arrangement almost every conceivable condition may occur 

 (fig. 349), brachiocephalic arteries on one or both sides being common. 



The renal portal system (best developed in the monotremes) has but a tran- 

 sitory existence, disappearing early with the degeneration of the mesonephroi 



Fig. 348. — A, Heart and adjacent vessels of swan, after Gadow; 5, renal circulation 

 of bird, after Gegenbaur. ao, aorta; c, carotid; ca, ccciiac artery; ci, caudal vein; cm, 

 coccygeo-mesenteric vein; cr, crural vein; A, hypogastric vein; /;, left jugular; 0, oesopha- 

 gus; ot, CESophageal artery; p, pulmonary artery; pc, precava; /><?, postcava; r;', right 

 jugular; sc, subclavian artery; t, trachea; ih, thoracic artery; Ir, thyreoid; v, vertebral 

 artery. 



(Wolffian bodies). As these organs disappear a part of the capillary system of 

 the Wolffian bodies enlarges and fornts a main trunk connecting the postcava 



ihi^ 



Fig. 349. — Modifications of the origin of the carotid and] subclavian arteries in 



mammals. 



with the posterior parts of the postcardinal veins (fig. 350, C) which bring the 

 blood from the tail, the iliacs and the permanent kidneys. With farther develop- 

 ment (Z), E) the left postcardinal is largely lost (except the part connecting" with 



