THE CIRCULATION OF THE BLOOD. 215 



tracting so as to diminish, and of passively dilating or yielding so as to 

 permit an increase of, the supply of blood, according to the requirements 

 of the part to which they are distributed. And thus, while the ventri- 

 cles of the heart determine the total quantity of blood, to be sent on ward 

 at each contraction, and the force of its propulsion, and while the large 

 and merely elastic arteries distribute it and equalize its stream, the 

 smaller arteries, in addition, regulate and determine, by means of their 

 muscular tissue, the proportion of the whole quantity of blood which 

 shall be distributed to each part. 



This regulating function of the arteries is governed and directed by 

 the nervous system in the way to be presently described. 



The muscular element of the middle coat also co-operates with the 

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

 which they contain. For the amount of fluid in the blood-vessels varies 

 very considerably even from hour to hour, and can never be quite con- 

 stant; and were the elastic tissue only present the pressure -exercised 

 by the walls of the containing vessels on the contained blood would be 

 sometimes very small, and sometimes inordinately great. The presence 

 of a muscular element, however, provides for a certain uniformity in the 

 amount of pressure exercised; and it is by this adaptive, uniform, gen- 

 tle, muscular contraction, that the normal tone of the blood-vessels is 

 maintained. Deficiency of this tone is the cause of the soft and yield- 

 ing pulse, and its unnatural excess of the hard and tense one. 



The elastic and muscular contraction of an artery may also be re- 

 garded as fulfilling a natural purpose when, the artery being cut, it first 

 limits and then, in conjunction with the coagulated fibrin, arrests the 

 escape of blood. It is only in consequence of such contraction and co- 

 agulation that we fire free from danger through even very slight wounds ; 

 for it is only when the artery is closed that the processes for the more 

 permanent and secure prevention of bleeding are established. But there 

 appears no reason for supposing that the muscular coat assists, to more 

 than a very small degree, in propelling the onward current of blood. 



The Pulse. 



The most characteristic feature, then, of the arterial flow, is its in- 

 termittency, and this intermittent flow is seen or felt as the Pulse. 



The pulse is generally described as an expansion of the artery pro- 

 duced by the wave of blood set in motion by the injection of blood at 

 each ventricular systole into the already full aorta. As the force of the 

 left ventricle, however, is not expended in dilating the aorta only, the 

 wave of blood passes on, expanding the arteries as it goes, running as it 

 were on the surface of the more slowly travelling blood already con- 

 tained in them, and producing the pulse as it proceeds. 



