BLOOD SYSTEM. 



387 



the head, unite to form the abdominal aorta which goes 

 to supply the viscera and lower portion of the trunk and 

 limbs. 



I have taken the lizard as a type both of the three- 

 chambered heart and of the structure of the aortic arches. 

 But there is considerable variation among reptiles in 

 both of these, and these 

 variations show transitions 

 such as one would expect 

 on the theory of derivative 

 origin of organic forms. 

 For example, the perfect 

 three-chambered heart is 

 general among reptiles, as 

 in lizards and tortoises, 

 and also in all amphibians 

 — e. g., in frogs, toads, etc. ; 

 but in serpents there is an 

 imperfect four-chambered 

 heart. The ventricle has 

 been partly but not com- 

 pletely divided. In the 

 crocodile there is a com- 

 plete fmcr-chambered heart, 

 but still the blood is mixed 

 in the course of the circu- 

 lation, though not in the 

 heart. Also the arches are 

 more or less modified from 

 the type given above. 



Fishes. — Fishes have a still simpler, viz., a two- 

 chambered heart, and yet the whole of the blood passes 

 through the gills to be aerated, and therefore pure blood 

 only goes to the tissues. Fig. 267 is a schematic dia- 

 gram showing the general course of the circulation in a 



Fig. 266. — Showing heart and out- 

 going blood vessels of a lizard. 

 The arrows show the course of 

 the blood. (After Owen.) 



