174 FOUNDATIONS OF BIOLOGY 



there is a continuous drainage of lymph from the capillaries 

 into intercellular lymph spaces. Some of the fluid, with waste 

 products, etc., passes immediately into the capillaries again, 

 but the excess passes from lymph spaces into small lymph 

 vessels, and from these into large lymph vessels. The latter, 

 in turn, empty into the venous system and so restore the 

 materials to the blood. (Fig. 93.) 



So much for the path and the duties of the liquid tissue 

 which circulates through the body. But clearly some provi- 

 sion must exist for regulating the blood flow in order to meet 

 the varying local demands of the organs of the body under 

 different physiological conditions. This is attended to chiefly 

 by nerve impulses which are conducted by a system of VASO 

 MOTOR nerves and bring about the dilation or contraction of 

 the smaller blood vessels (arterioles) leading to an organ, and 

 thus increase or decrease the volume of the blood which it 

 receives. The elaborate mechanism in homothermal animals, 

 which maintains a practically constant body temperature, 

 is largely dependent upon heat distribution, loss, and con- 

 servation by the blood vascular system. Since the total 

 volume of blood in the body is practically constant, an extra 

 supply to one part obviously necessitates a reduced supply 

 to another. So it happens, for instance, that after a hearty 

 meal more blood is concentrated where digestion is actively 

 going on, leaving less for the other organs the reduced 

 supply to the brain resulting in the proverbial drowsiness 

 at such times. 



