90 



THE CIRCULATION. 



named, it will be noticed also in the same figure, that there is 

 another, by which a portion of the stream of blood having 

 been diverted once into the capillaries of the intestinal canal, 

 and some other organs, and gathered up again into a single 

 stream, is a second time divided in its passage through the 



FIG. 32. 



Diagram of the circulation. 



liver, before it finally reaches the heart and completes a revo- 

 lution. 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) 



