IV. VERTEBRATA. 



549 



ducts and jugular and cardinal veins, the latter with growth en- 

 croaching more and more into the territory of the subintestinal 

 vein. 



The circulation of the fish type undergoes a great modification 

 with the loss of gills and the appearance of pulmonary respiration. 

 Gills and gill capillaries disappear, and the branchial circulation is 

 reduced to arterial arches leading direct from the ventral to the 

 dorsal aorta. The swim bladder received its blood from the body 

 (systemic) circulation, but with the functioning of the lungs pul- 

 monary arteries and veins come into existence, while the arterial 

 arches in part disappear, in part are divided between the pulmonary 

 I II 111 IV 



FIG. 580. Diagram of modification of arterial arches in various vertebrate classes. White, 

 vessels which degenerate; cross-lined, vessels containing arterial blood; black, vessels 

 containing venous blood. /, Dipnoi; //, Urodeles with pulmonary respiration; HI, 

 Reptiles: IV, Birds (in mammals the left instead of the right aortic arch persists), ao 1 , 

 venous aorta of reptiles; ao 2 , arterial aorta; ast, arterial trunk; a, b, arches which 

 usually disappear: ad, dorsal aorta: d.B. ductus Botalli; fc, gill capillaries; pu, pul- 

 monary artery; 1-k, persistent arterial arches. 



and systemic circulations (fig. 580). Of the six arches which 

 usually appear in the embryo, the first and second, and the fifth 

 in animals with lungs, usually disappear. The last arch (4), which 

 even in the Dipnoi supplies the swim bladder, becomes a pulmonary 

 artery, the other arches (1 and 2) furnish the systemic portions, 

 the dorsal aorta (2) and the carotids supplying the head (1). 

 Since special pulmonary veins, distinct from the systemic circula- 

 tion, carry the blood from the lungs to the heart, the heart be- 

 comes divided by a septum which separates it into right and left 

 halves. The right half retains the venous character of the fish 

 heart; since the right auricle receives the systemic veins, the right 

 ventricle gives off the pulmonary artery. The left half is purely 

 arterial, receiving arterial blood by the left auricle from the lungs 

 and sending it out through the aorta ascendens to the body. A 

 complete separation of pulmonary and systemic circulation, and a 

 corresponding division of the heart, occurs only in birds and mam- 



