THE CIRCULATION 



263 



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 estab- 

 lished, but certain drugs 

 appear to act specially upon 

 them. Thus apocodeine, 

 while it does not prevent 

 barium salts from constrict- 

 ing the vessels, prevents the 

 constricting action of ex- 

 tracts of the medulla of 

 the suprarenals, even when 

 the nerves are cut ; and 

 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. Yaso-motor Nerves. 



When a nerve going to any FlG . i 2 6.-Diagram of the Distribution of 

 part of the body is cut the 

 arterioles of the part gene- 

 rally dilate, when it is stimu- 

 lated the arterioles are 

 usually contracted ; some- 

 times, however, they are 

 dilated. In no case does section of a nerve cause constric- 

 tion of the arterioles. 



These facts prove that the vaso-motor nerves may be 

 divided into two classes: 



1st. Vaso-constrictor. 



Vaso-motor Nerves. The continuous 

 line shows the Vaso-constrictors ; the 

 dotted line the Vaso-dilators. C.N., 

 Cranial Nerves; Vag., Vagus; T.S., 

 Thoracic Sympathetic ; A. St., Ab- 

 dominal Sympathetic; N.L., Nerves 

 to the Leg. 



