262 HUMAN PHYSIOLOGY 



muscle fibres. This rhythmic action is better marked in 

 some vessels than in others. 



Yaso-motor Mechanism. If the sciatic nerve of a frog be 

 cut, the arterioles in the foot at once dilate. If the sciatic is 

 stimulated, the arterioles become smaller. The same results 

 follow if the anterior roots of the lower spinal nerves, from 

 which the sciatic takes origin, be cut or stimulated. 



We must, therefore, conclude (1) that the central nervous 

 system exerts a constant tonic influence upon the arterioles, 

 keeping them in a state of semi-contraction ; and (2) that this 

 influence may be increased, and thus a constriction of the 

 arterioles caused, and in this way the flow of blood from 

 arteries to capillaries obstructed and the arterial pressure 

 raised ; and (3) that this influence may be diminished, so 

 that the arterioles dilate and allow an increased flow into the 

 capillaries from the arteries, and thus lower the arterial 

 pressure. 



These mobile arterioles, under the control of the central 

 nervous system, constitute a vaso-motor mechanism, which 

 plays a most important part in connection with nearly every 

 vital process in the body. By it the pressure in the arteries 

 is governed, by it the supply of blood to the capillaries and 

 tissues is controlled, and by it the loss of heat from the body 

 is largely regulated. 



This vaso-motor mechanism consists of the three parts : 



1st. The contractile muscular walls of the arterioles with 

 the nerve terminations in them. 



2nd. The nerves which pass to them. 



3rd. The portions of the central nervous system presiding 

 over these. 



1. Muscular Walls of the Arterioles. The muscular fibres 

 are maintained in a state of tonic semi-contraction by nerves 

 passing to them, and when these nerves are divided, the 

 muscular fibres relax. But if, after these nerves have been 

 cut, the animal be allowed to live, in a few days the arterioles 

 again pass into a state of tonic semi-contraction, although 

 no union of the divided nerve has taken place. 



Certain drugs, e.g. digitalis and the salts of barium, act as 

 direct stimulants to these muscle fibres. 



