GENERAL CHARACTERS OP REPTILES. 545 



We have elsewhere seen (ANIM. PHYSIOL. 289), that among 



brl 



Pio. 323. The same, in a more advanced state ; the communicating branches are 

 now enlarged, so that much of the hlood transmitted to the branchial arteries finds its 

 way at once into the branchial veins, without passing through the gills ; the pulmonary 

 arteries, ap, are also much increased in size. 



Fishes, the blood follows the same course ; but when the lungg 

 are developed, the arrange- 



ment of the vascular appa- 

 ratus changes; there is esta- 

 blished a direct communi- 

 cation between the arteries 

 which carry the blood to 

 the gills, and those which 

 return it from these organs ; 

 so that the nutritious fluid 

 is no longer compelled to 

 traverse this apparatus of 

 aquatic respiration, in order 

 to arrive at the dorsal artery, 



6K1 



ap 



FIG. 324. The same, in the perfect animal ; 

 the gills have now disappeared, and the course 

 of the blood is entirely through the communi- 

 cating branches ; that of the first pair conveys 

 it direct to the trunks, t, t, supplying the head ; 

 and those ( of the second and third to the aorta, 

 av. The respiratory circulation is now entirely and which might be at first 



compared to a branchial 

 artery, then becomes the origin of the dorsal trunk, and forms 



and thence into the different 

 parts of the body (Fig. 

 323). The artery (a) which 

 arises from the ventricle, 



