THE CIRCULATION 143 



more to an intermittent stream, and the pulse wave is 

 propelled more rapidly throughout the body. It is for 

 this reason that, in cases of aortic incompetence which 

 are secondary to aortic atheroma, the * delayed ' pulse 

 characteristic of incompetence due to rheumatic endo- 

 carditis is not observed. It should be noted, also, that 

 the distensibility of the elastic coat of the arteries is 

 greatest when the pressure within them is moderate. 

 In conditions of high tension a greater degree of force 

 is required to stretch them, and hypertrophy of the 

 heart results, whilst any increase in the pulse volume 

 raises the tension within the arteries far more than it 

 would do at a lower pressure, and so tends to rupture 

 them. 



In the smaller arteries the muscular element pre- 

 dominates over the elastic fibres, for it is in the smaller 

 arteries that the power of actively regulating the amount 

 of blood-flow to any part resides. This stop-cock action 

 of the arterioles will be more fully considered under the 

 head of blood-pressure, and is largely controlled by the 

 vasomotor nerves, but it may be pointed out here that 

 the muscular wall has a certain amount of independent 

 action. Thus, the muscular coat of the small arteries 

 reacts to a stretching force of contraction* just like 

 unstriped muscle elsewhere. Hence any permanent 

 increase in the volume of the blood may be expected 

 to lead to hypertrophy of the muscular coat, and in con- 

 ditions of plethora this actually occurs. The muscular 

 coat also contracts to cold or great degrees of heat, an 

 action which is constantly taken advantage of in the 



* Bayliss, Journ. of Physiol, 1902, vol. xxviii. 220. 



