THE ESSENTIAL PHENOMENA OF CIRCULATION. 



25 



FIG. 13. 



the large trunks of the arterial and venous circulation ; 

 the first opens from the left side, the second opens into the 

 right side, and we might 

 say that the left heart, is 

 an arterial heart, while 

 the right is a venous 

 heart. If this be remem- 

 bered, no mistake can be 

 made in understanding 

 the course of the blood 

 in the circulation. We 

 must also remember that 

 the two hearts are abso- 

 lutely separated by a mid- 

 dle partition, and that 

 the blood can never pass 



directly from the one to DIAGRAM OP HEART HALVED AND LAID 

 the other, also that the 

 blood in either auricle 

 can flow only into the 

 corresponding ventricle. 



It naturally follows 

 from this arrangement 

 that the blood contained in either ventricle, being unable 

 to escape into the auricle on account of the valve, and 

 also unable to enter the other ventricle, must be forced 

 into the vessels. 



The heart then acts as follows : 



It is the pumping organ of the circulatory system. 

 Its contractions communicate to the blood which passes 

 through it a motion that is continued throughout the 

 whole extent of the vascular system. The mechanism 

 must be studied at two distinct intervals. At first there 

 B 3 



OPEN (after Debierre). A, B, C, D, as in 

 Fig. 12. a, part of tricuspid valve ; 6, 

 part of mitral ; c, semilunars at base of 

 pulmonary artery, a', a', inferior and 

 superior venae cavse entering A ; 6', 6', 

 pulmonary arteries proceeding from B ; 

 c?, c', aorta proceeding from C ; d', d' t 

 pulmonary veins entering D. 



