CHAPTER XVI. 



FIG. 110. 



R.H. 



L.H, 



THE HEAKT. 



THE course taken by the blood on its way to the various parts 

 of the body is called the circulation, on account of its having to 

 make repeatedly the circuit of vessels leading from and to the 

 heart. The heart is the great motor power which drives the blood 

 through all the vessels, of which there is one set leading from 

 and to the organs of the system generally, and another set lead- 

 ing to and from the lungs. 



Anatomists speak of two circulations the greater or systemic, 

 and the lesser or pulmonary. How- 

 ever, if we follow the course of the 

 blood, we see that both these sets 

 of vessels really belong to the one 

 circulation, and in fact form but 

 one circuit. The blood passing 

 through the lungs and systemic 

 vessels consecutively visits the 

 heart twice on its way, in order to 

 acquire the force necessary to 

 overcome the resistance of the two 

 sets of capillaries. Although in 

 all the higher animals the heart 

 forms but a single organ, it prac- 

 tically is composed of two muscular pumps which are anatomically 

 united but distinct in function. In its course round the circula- 

 tion the blood visits each of these functionally distinct hearts at 

 quite different parts of its circuit. The right heart is placed 

 before the pulmonary vessels and pumps the blood through the 

 lungs. The left heart is placed before the systemic vessels and 

 pumps the blood through the body generally. Thus anatomically 

 there appear to be two circulations and but one heart; physio- 



Diagram of Circulation, show- 

 ing right (E.H.) and left (L.H.) 

 hearts, and the pulmonary (p) and 

 systemic (s) sets of capillaries. 



