THE HEART. 91 



the interchange of relations between the blood and the tissues 

 which ensues in the capillary system during the nutritive pro- 

 cesses. The right direction of the blood's course is determined 

 and maintained by the valves of the heart to be immediately 

 described ; which valves open to permit the movement of the 

 blood in the course described, but close when any force tends 

 to move it in the contrary direction. 



We shall consider separately each member of the system of 

 organs for the circulation : and first 



The Heart. 



The heart is a hollow muscular organ, the interior of which 

 is divided by a partition in such a manner as to form two 

 chief chambers or cavities right and left. Each of these 

 chambers is again subdivided into an upper and a lower por- 

 tion called respectively the auricle and ventricle, which freely 

 communicate one with the other ; the aperture of communica- 

 tion, however, being guarded by valvular curtains, so disposed 

 as to allow blood to pass freely from the auricle into the ven- 

 tricle, but not in the opposite direction. There are thus four 

 cavities altogether in the heart two auricles and two ventri- 

 cles ; the auricle and ventricle of one side being quite sepa- 

 rate from those of the other. The right auricle communicates, 

 on the one hand, with the veins of the general system, and, 

 on the other, with the right ventricle, while the latter leads 

 directly into the pulmonary artery, the orifice of which is 

 guarded by valves. 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 sys- 

 tem, 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 pulmonary artery, by which 

 it is conveyed to the capillaries of the lungs. From the lungs 

 the blood, which is now purified and altered in color, is ga- 

 thered by the pulmonary 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 capil- 

 laries of every portion of the body. The branches of the aorta, 

 from being distributed to the general system, are called sys- 

 temic arteries ; and from these the blood passes into the systemic 



