INNERVATION OF VESSELS 51 



the lymph. The flow in the capillaries is constant, as we have 

 seen, and we can see the importance of this when we take into 

 consideration how rapidly the tissues use oxygen and how 

 necessary is a constant increasing supply, and how essential it is 

 to remove the carbon dioxide poisons. 



Innervation of Vessels. The blood-vessels are controlled 

 by the sympathetic nervous system by means of the vaso-motor 



FIG. 26. 



A, vein with valves open. B, with valves closed; stream of blood passing off by 

 lateral channel. (Kirkes after Dalton.) 



nerves. These compose both the vaso-constrictors, or the 

 nerves causing the vessels to contract, and the vaso-dilators, 

 those causing the vessels to dilate. The entire physiological dis- 

 tribution of blood is regulated by the vaso-motor system of nerves. 

 It is by their means that the blood is increased to all parts of the 

 body where physiological activity is going on, as when the gastro- 

 intestinal tract is active during digestion, a muscle in motion, or 

 glands in activity. Paralysis of the vaso-constrictors causes 

 blushing, paralysis of the dilators causes pallor as from fright. 



