CH. XXI.] THE PULSE. 267 



tube, assists in the conversion of the intermittent into a continuous 

 stream, is to be found in the arterioles or small arteries, just 

 before the blood passes into what we may term the vast capillary 

 lake. These small arteries with their relative excess of muscular 

 tissue, which in health is always in a tonic state of moderate 

 contraction, play the part of a multitudinous system of stop- 

 cocks. 



The large arteries contain a considerable amount of muscular 

 as well as elastic tissue. This co-operates with the elastic tissue 

 in adapting the calibre of the vessels to the quantity of blood 

 they contain. For the amount of blood in the vessels is never 

 quite constant, and were elastic tissue only present, the pressure 

 exercised by the walls of the containing vessels on the contained 

 blood would be sometimes very small, sometimes too great. The 

 presence of a contractile element, however, provides for a certain 

 uniformity in the amount of pressure exercised. There is no 

 reason to suppose that the muscular coat assists in propelling 

 the onward current of blood, except in virtue of the fact that 

 muscular tissue is elastic, and therefore co-operates in the large 

 arteries with the elastic tissue in keeping up the constant flow in 

 the way already described. 



The contractility of the arterial walls fulfils a useful purpose 

 in checking haemorrhage should a small vessel be cut as it assists 

 in the closure of the cut end, and this in conjunction with the 

 coagulation of the blood arrests the escape of blood. 



The Pulse. 



This is the most characteristic feature of the arterial flow. It 

 is a wave of expansion which travels along the arteries due to the 

 propulsion of the contents of the left ventricle into the already 

 full arterial system. The more distant the artery from the heart, 

 the longer is the interval that elapses between the ventricular 

 beat and the arrival of the pulse. Thus it is felt in the carotid 

 earlier than in the radial artery, and it is still later in the dorsal 

 artery of the foot. The difference of time is, however, very 

 slight ; it is only a minute fraction of a second ; for a distinction 

 must be drawn between the propagation of the pulse and the rate 

 of blood flow in the arteries ; the wave travels at the rate of from 

 5 to 10 metres a second, that is twenty or thirty times the rate 

 of the blood current. The pulse may be compared to a wave 

 produced by the wind travelling rapidly down a sluggishly-flowing 

 river. 



A physician usually feels the pulse in the radial artery, since 



