THE HEART. 113 



The left auricle again communicates, on the one hand, with 

 the pulmonary veins, and, on the other, with the left 

 ventricle, while the latter leads directly into the aorta a 

 large artery which conveys blood to the general system, 

 the orifice of which, like that of the pulmonary artery, is 

 guarded by valves. 



The arrangement of the heart's valves is such that 

 the blood can pass only in one definite direction, and 

 this is as follows (fig. 33): From the right auricle 

 the blood passes into the right ventricle, and thence 

 into the pulmo- Fig. 33.* 



nary artery, by 

 which it is con- 

 veyed to the ca- 

 pillaries of the 

 lungs. From the 

 lungs the blood, 

 which is now puri- 

 fied and altered 

 in colour, is ga- 

 thered by the pul- 

 monary veins and 

 taken to the left 

 auricle. From the 

 left auricle it 

 passes into the 

 left ventricle, and 

 thence into the 

 aorta, by which it is distributed to the capillaries of every 

 portion of the body. The branches of the aorta, from 

 oeing distributed to the general system, are called systemic 

 arteries ; and from thence the blood passes into the sys- 

 temic capillaries, where it again becomes dark and impure, 

 and thence into the branches of the systemic veins, which, 



* Fig. 33. Diagram of the circulation through the heart (after Dalton). 



T 



