CIECULATION OF THE BLOOD. 



253 



FIG. 112. 



RA. 



P.V. 



About three hundred years ago the true course of the blood 

 current through the systemic and pulmonary heart, arteries, and 

 veins, so as to form one circle, was demonstrated by Harvey. 

 Before his time only the so-called " lesser " or pulmonary circuit 

 was known. But the magnifying glasses at his disposal did not 

 enable him to see the capillaries 

 which were first described by Mal- 

 pighi some fifty years later. 



In the hope of making the gen- 

 eral differences of functions more 

 striking, the various parts of the 

 circulatory apparatus may be enu- 

 merated according to their several 

 duties and roughly illustrated by 

 a diagram (Fig. 112) : 



1. The left (systemic) heart (L. 

 H.) pumps the blood into the sys- 

 temic arteries, and thus keeps these 

 vessels over-filled. 



2. The larger systemic arteries 

 (A.), by their elasticity exert con- 

 tinuous pressure on the blood with 

 which they are distended. 



3. The smaller systemic arterioles 

 (A'.), by their vital contractility, 

 check and regulate the amount of 

 blood flowing out of the larger 

 arteries into the capillaries, and 

 thus keep up the tension of the 

 larger arteries. 



4. The systemic capillaries (S. C.), where the essential opera- 

 tion of the blood is carried out, viz., the interchange between it 

 and the tissues. 



5. The wide systemic veins (V.) are the passive channels con- 

 veying the impure blood to the pulmonary heart. 



6. The right (pulmonary) heart (R. H.), pumps the blood into 

 the pulmonary arteries and over-fills them. 



s.c. 



Diagram of the Circulation of 

 the Blood and the absorbent ves- 

 sels. For details see text. 



