454 VETERINARY PHYSIOLOGY 



coat of the arterioles to relax, and thus, by dimmishing 

 peripheral resistance, permit blood to flow freely from the 

 arteries into the capillaries. 



Salts of barium have precisely the opposite effect, causing 

 the skin to become pale from imperfect filling of the 

 capillaries, and producing a marked rise in the arterial 

 pressure. Contraction of the muscles of the arterioles is 

 produced, and the flow of blood from arteries to capillaries 

 is retarded. 



Histamine seems to have the peculiar action of constrict- 

 ing the arterioles, and in some animals at least of dilating 

 the capillaries. 



(8) Nervous Mechanism Controlling the Arterioles- — If the 

 sciatic nerve is cut, the small vessels in the foot at once 

 dilate. If the nerve is stimulated, they contract. The same 

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

 from which the sciatic takes origin, are first cut and then 

 stimulated. 



T/te central nervous system, therefore, exerts a constant 

 tonic influence ujjon the arterioles, keeping them in a state 

 of semi-contraction, and this action may be increased, and 

 thus a constriction of the arterioles produced. In this way, 

 if the eftect is a general one, the flow of blood from arteries 

 to capillaries is obstructed and the arterial pressure may be 

 raised. This influence may also be diminished, so that the 

 arterioles dilate and allow an increased flow into the 

 capillaries from the arteries. Thus the arterial pressure 

 may be lowered if the action is not too local, and is not 

 compensated for by constriction elsewhere. 



These mobile arterioles, under the control of the central 

 nervous svstem, constitute a vaso-motor mechanism, which 

 plays a part in nearly every vital process in the body. By 

 it the pressure in the arteries is governed, the supply of 

 blood to the capillaries and tissues is controlled, and the loss 

 of heat from the skin is largely regulated (p. 269). 



This vaso-motor mechanism consists of two parts : — 



1st. Peripheral. — This consists of the muscular fibres in 

 the walls of the arterioles with the nerve structures amonsf 

 them. 



