ii2 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 some 

 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 or 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 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. 



