124 



PRINCIPLES OF ANIMAL BIOLOGY 



reptiles, except crocodilians, has three chambers in place of two as in the 

 heart of fishes (Fig. 97), and the heart of mammals, birds, and croco- 

 dilians has four chambers. The four-chambered heart is composed of 

 two halves, right and left. Each half is made up of two chambers, a 

 thin-walled auricle and a thick-walled muscular ventricle. There is no 

 passage between the two halves of the heart but there is a broad passage 

 guarded by valves connecting each auricle with the ventricle of the same 

 side. The relations of the parts of a four-chambered heart may be 

 understood from Fig. 98. 



The circulation in such an animal is a double one. Beginning at the 

 left ventricle (see Fig. 99 for the human scheme), the blood is driven 

 into the large artery which, with its divisions, leads to the body in 

 general, including the head. In these parts the arteries divide into 



capillaries, which are collected again into 

 veins. The veins gather into two large veins 

 which enter the heart by the right auricle. 

 The circuit just described from left ventricle 

 through the body to right auricle, is called 

 the systemic circulation. The blood now goes 

 from the right auricle, through valves, to the 

 right ventricle, thence is forced to the lungs. 

 After passing through the capillaries of the 

 lungs it returns by a large vein to the left 

 auricle of the heart, thence to the left ventricle. 

 The circuit through the lungs is called the 

 'pulmonary circulation. In a complete circula- 

 tion, therefore, the blood passes through the 

 heart twice, once through the left side, once 

 through the right. The blood has no alter- 

 native in this course, except that in the 

 systemic circulation it may go to any one of a number of parts of the 

 head, trunk, extremities, or abdominal organs. When it has gone 

 through the systemic circuit, it has no choice but to go to the lungs. 



The doubleness of this circulation is a consequence of the four-cham- 

 bered heart, that is, of its complete separation into right and left halves. 

 In animals with a three-chambered heart, as in a frog, this distinctness 

 does not prevail, for while there are two auricles there is but a single 

 ventricle. There is therefore some mixing of the l)lood in the ventricle; 

 but the structure of the ventricle with its deep recesses and the operation 

 of valves in the principal artery are such that the mixing of venous and 

 arterial blood is partially prevented. 



In general, when the heart has four chambers, the blood passes 

 through only one set of capillaries in each circuit. There is only one set 



Fig. 98. — Diagram of a 

 four-chambeied heart. LA, 

 left auricle; RA, right auricle; 

 LV, left ventricle; RV, right 

 ventricle; L\, vessel from 

 lungs; L2, vessel to lungs; S\, 

 vessel to system; Si, vessel 

 from system. 



