484 



CHORDATA. 



veins, consisting of Cuvierian ducts and jugular and cardinal veins, the 

 latter with growth encroaching 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 in the higher 

 vertebrates. 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 pulmonary 

 arteries and veins come into existence, while the arterial arches in part 

 7 77 777 IV 



p IG- 5^7. 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. 7, Dipnoi; 77, Urodeles with pulmonary respiration; 

 777, Reptiles; IV, Birds (in mammals the left instead of the right aortic arch persists). 

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

 usually disappear; ad, dorsal aorta; d.B, ductus Botalli; k, gill capillaries; pu, pulmo- 

 nary artery; 1-4, persistent arterial arches. 



disappear, in part are divided between the pulmonary and systemic 

 circulations (fig. 537). Of the six arches which usually appear in the 

 embryo, the first and second, and the fifth in animals with lungs, usually 

 degenerate. The last arch (4), which even in the Dipnoi supplies the 

 swim bladder, becomes a pulmonary artery, the other arches (i and 2) 

 furnish the systemic portions the dorsal aorta (2) and the carotids supply- 

 ing the head (i). Since special pulmonary veins, distinct from the systemic 

 circulation, 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. This complete separation of pul- 

 monary and systemic circulation, and the corresponding division of 



