ir. VERTEBRATA. 



f/49 



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

 croaching more and more into tlie territory of the subintestlnal 

 vein. 



Tlie circulation of the fish type undergoes a great modiilcation 

 with the loss of gills and the appearance of imlmonary 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 HI IV 



Fig. 5R0 — Diagram of modification of arterial arches in various vertebrate classes. White, 

 vessels which det^enerate; cross-lined, vessels containing arterial blood: black, vessels 

 containing venous blood. 7, Dipnoi; //, Urodeles with pulmon;jry respiration; 111, 

 Ilp)itiles: /r. Birds (in niamaials the left instead of the right aortic arch persists). «o^ 

 venous aorta of reptiles: (fo^, arterial aorta: ast, arterial trunk; a.h, arches which 

 usually disappear: ad, dorsal artrta: <l.H. ductus Botalli; A, gill capillaries; pu, pul- 

 monary artery: 1-U, persistent arterial arches. 



and systemic circulatioi^s (fig. 580). Of tlie six arclies wliich 

 usually apjK-ar in tlie emliryo, the first and second, aud the fifth 

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

 even in the Dipnoi supplies the swim liladder, becomes a jiulmonary 

 artery, the other arches {1 and 2) furnish the systemic jiortions, 

 tlie dorsal aorta [2) and the carotids supplying the head (i). 

 Since special pulmonary veins, distinct from the S3'stemic circula- 

 tion, carry the blood fr&m 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 sejiaration of jiulmonary and systemic circulation, and a 

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



