252 



MANUAL OF PHYSIOLOGY. 



FlG. 111. 



logically there is one circulation and two hearts ; or two points of 

 resistance and two distinct pumping organs to drive the blood 

 through the obstacles. 



The circulation might then be represented by a simple diagram 

 (Fig. 110) in which the direction of the current is indicated by 



the arrows. L. H. shows the 

 position of the left or syste- 

 mic pump, and S. the resist- 

 ance in the systemic vessels. 

 R. H. represents the pul- 

 monary pump, and P. the 

 second obstacle in the cir- 

 cuit, viz., the vessels of the 

 lungs. 



However, it must be re- 

 membered that the right 

 and left pumping organs are 

 fused into one viscus, which 

 has two distinct and separate 

 channels for the passage of 

 the blood through it. In 

 each system of bloodvessels 

 we have the same general 

 arrangement for the distri- 

 bution and re-collection of 

 the blood. 



In passing from either the right or left side of the heart the 

 blood flows into tubes called arteries, which divide and subdivide 

 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 

 vessels 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 bloed 

 back to the heart. 



Capillary Network of the Choroid of a 

 Child of a few months old. (Cadiat.) 

 (a) Artery. (6) Vein, and capillary net- 

 work intervening. 



