102 THE CIRCULATION. 



before it finally reaches the heart and completes a revolu- 

 tion. This subordinate stream through the liver is called 

 the portal circulation. 



The principal force provided for constantly moving the- 

 blood through this course is that of the muscular substance- 

 of the heart ; other assistant forces are (2) those of the 

 elastic walls of the arteries, (3) the pressure of the muscles 

 among which some of the veins run, (4) the movements of 

 the walls of the chest in respiration, and probably, to soma 

 extent, (5), the interchange of relations between the blood 

 and the tissues which ensues in the capillary system during 

 the nutritive processes. 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 as cavities right and left. Each 

 of these chambers is again subdivided into an upper and 

 a lower portion called respectively the auricle and ventricle, 

 which freely communicate one with the other ; the aperture 

 of communication, however, being guarded by valvular 

 curtains, so disposed so as to allow blood to pass freely from 

 the auricle into the ventricle, but not in the opposite direc- 

 tion. There are thus four cavities altogether in the heart 

 two auricles and two ventricles ; the auricle and ventricle- 

 of one side being quite separate 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 pul- 

 monary artery, the orifice of which is guarded by valves. 



