CHAPTER XVI. 



THE HEART. 



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 to and from 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 

 leading 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 on its way R>H - W} Til L -H. 



through the lungs and the systemic 

 vessels visits the heart twice, in 

 order to acquire the force necessary 

 to overcome the resistance of the Diagram of Circulation ghow|ng 



tWO Sets Of Capillaries. In all the right (R.H.) and left (L. H.) hearts, and 



higher animals the heart forms but the P lraon y (p) andsystemic (s) sets 



. . of capillaries. 



a single organ, but practically it is 



composed of two muscular pumps which are anatomically united, 

 but are distinct in function. These functionally distinct hearts 

 work at different parts of the circuit traveled by the blood. The 

 right heart is placed before the pulmonary vessels and pumps the 

 blood through the lungs. The left heart is placed before the sys- 

 temic vessels and pumps the blood through the body generally. 

 Thus anatomically there appear to be two circulations and but 

 one heart ; physiologically, there is one circulation and two hearts ; 

 or two points of resistance and a distinct pumping organ to drive 

 the blood through each. 



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