CIRCULATION OF THE BLOOD. 259 



until the branches become microscopical in size. From the very 

 minute arteries the blood passes into the capillaries, which can- 

 not be said to branch, but are arranged so as to form a network 

 of delicate tubes with more or less close meshes, according to the 

 part. 



Connected with the meshes of the capillaries are other small ves- 

 sels which collect the blood from the networks (Fig. 111). These 

 unite one with another to form larger vessels, which, again, are but 

 the tributaries of the larger veins which bear the blood back to 

 the heart. 



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 

 Malpighi some fifty years later. 



In the hope of making the general differences of functions 

 more striking, the various parts of the circulatory apparatus may 

 be enumerated 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 

 systemic arteries, and thus keeps these vessels overfilled. 



2. The larger systemic arteries (A.), by their elasticity, exert 

 continuous pressure on the blood with which they are distended. 



3. The smaller systemic arterioles (A'.), by their vital con- 

 tractility, check and regulate the amount of blood flowing out of 

 the larger arteries into the capillaries, and thus keep up the ten- 

 sion 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 overfills them. 



