THE CIRCULATION OF THE BLOOD 



315 



by a vertical partition into two distinct chambers, each 

 of which is again divided by a valve into an auricle 

 and a ventricle. The work of the right auricle and 

 ventricle is to receive the blood from the veins, and 

 send it to the lungs ; while the other two receive 

 the blood from the lungs, and propel it over the body. 

 The left ventricle has more work to do than any of 



/ g 



FIG. 271. Theoretical Section of the 

 Human Heart: a, right ventricle; 

 b, inferior vena cava; c, tricuspid 

 valve; d, right auricle; e, pulmonary 

 veins; _/, superior vena cava; g, pul- 

 monary arteries; h, aorta; k, left 

 auricle; /, mitral valve; m, left ven- 

 tricle; n, septum. 



FIG. 272. Plan of Circula- 

 tion in Fishes: a, auricle; 

 b, ventricle; c, branchial 

 artery; e, branchial veins, 

 bringing blood from the 

 gills, d, and uniting in the 

 aorta,y/ g, vena cava. 



the other parts of the heart. The two auricles con- 

 tract at the same instant; so also do the ventricles. 

 The course of the current in birds and mammals is as 

 follows : the venous blood brought from the system is 

 discharged by two or three large trunks 115 into the right 

 auricle, which immediately forces it past a valve 116 into 

 the right ventricle. The ventricle then contracts, and 

 the blood is forced through the pulmonary artery past 

 its semilunar valves into the lungs, where it is changed 



