BLOOD VESSELS 451 



L Heart's Action.— The influence of this may be readily 

 demonstrated by stimulating the vagus nerve while taking a 

 tracing of the arterial pressure. The heart is inhibited, less 

 blood is forced into the arteries, and the pressure falls 



(fig- 188). 



If, on the other hand, the augnientor nerve is stimulated, 

 the increased heart's action drives more blood into the 

 arteries, and the pressure rises. 



IL Peripheral Resistance.— The resistance to outflow from 

 the arteries to the capillaries and veins depends upon (1) the 

 resistance offered in the small arteries, the walls of which 

 are surrounded by visceral muscle fibres. When these fibres 

 are contracted, the vessels are small and the resistance is 

 great. When they are relaxed, the vessels dilate, and the 

 resistance to outflow is diminished. This muscular tissue of 

 the arterioles acts as a stop-cock to the flow of blood from 

 the arteries to the capillaries. It is of great importance 

 — 1st, in maintaining the uniform pressure in the 

 arteries ; 2nd, in regulating the flow of blood into the 

 capillaries. 



(2) The condition of the capillaries. — Krogh has shown 

 that in resting muscle most of the capillaries are closed, 

 while in contracting muscle they are dilated and filled 

 with blood even when the arterial pressure has not 

 been allowed to rise. The capillaries thus seem able to 

 contract and expand. Recently Dale has shown that 

 the administration of histamine to dogs and monkeys 

 causes such a dilatation with, at the same time, a 

 contraction of the arterioles. Krogh also finds that a 

 dilatation of capillaries may be produced by stimulating 

 directly. 



He considers that the state of the arterioles regulates 

 the pressure of blood in the arteries, while the state 

 of the capillaries regulates the rate of flow. A slow 

 current through dilated capillaries means arteriole con- 

 striction. 



Dilatation of the arterioles and of the capillaries is gener- 

 ally local, and its effects upon the general arterial pressure is 



