CIRCULATION 279 



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 (p. 385). 



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 Artepioles. 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. 



It appears that the muscular fibres in the arterioles, as 

 elsewhere, tend to maintain themselves in a state of partial 

 contraction, which increases and diminishes in a regular 

 rhythmic manner. 



The precise part played by the nerve terminations has not 

 been definitely established, but certain drugs appear to act 

 specially upon them. Thus apocodeine, while it does not 

 prevent barium salts from constricting the vessels, prevents 

 the constricting action of extracts of the medulla of the 

 suprarenals, even when the nerves are cut. Hence it 

 must be concluded that it paralyses a nervous mechanism 

 in the arteriole wall which is stimulated by the suprarenal 

 extract. 



Normally this muscular mechanism is controlled by the 

 nervous system. 



2. Vaso-motor Nerves. When a nerve going to any part of 

 the body is cut the arterioles of the part generally dilate, 

 when it is stimulated the arterioles are usually contracted; 

 sometimes, however, they are dilated. In no case does section 

 of a nerve cause constriction of the arterioles. 



