THE CIRCULATION OF THE BLOOD. 213 



resistance, and thus the two altered conditions may balance, with the 

 result of bringing back the blood-pressure to what it was before the 

 heart began to beat more rapidly or more forcibly. 



It will be clearly seen that the circulation of the blood within the 

 blood-vessels must depend upon the diminution of the pressure from 

 the heart to the capillaries, and from the capillaries to the veins, the 

 blood flowing in the direction of least resistance; we shall presently see 

 further that the general or local flow also depends upon the relations 

 between the heart's action and the peripheral resistance, general or local. 



The Arterial Flow. 



The character of the flow of blood through the arterial system de- 

 pends to a very considerable extent upon the structure of the arterial 

 walls, and particularly upon the elastic tissue which is so highly devel- 

 oped in them. 



The elastic tissue first of all guards the arteries from the suddenly 

 exerted pressure to which they are subjected at each contraction of the 

 ventricles. In every such contraction as is above seen the contents of 

 the ventricles are forced into the arteries more quickly than they can 

 be discharged through the capillaries. The blood, therefore, being, for 

 an instant, resisted in its onward course, a part of the force with which 

 it was impelled is directed against the sides of the arteries; under this 

 force their elastic walls dilate, stretching enough to receive the blood, 

 and, as they stretch, becoming more tense and more resisting. Thus, 

 by yielding they break the shock of the force impelling the blood. On 

 the subsidence of the pressure, when the ventricles cease contracting, 

 the arteries are able, by the same elasticity, to resume their former cali- 

 bre; the elastic tissue also equalizes the current of blood by maintaining 

 pressure on it in the arteries during the period at which the ventricles 

 are at rest or are dilating. If the arteries were rigid tubes, the blood, 

 instead of flowing, as it does, in a constant stream, would be propelled 

 through the arterial system in a series of jerks corresponding to the 

 ventricular contractions, with intervals of almost complete rest during 

 the inaction of the ventricles. But in the actual condition of the ves- 

 sels, the force of the successive contractions of the ventricles is expended 

 partly in the direct propulsion of the blood, and partly in the dilatation 

 of the elastic arteries; and in the intervals between the contractions of 

 the ventricles, the force of the recoil is employed in continuing the on- 

 ward flow. Of course the pressure exercised is equally diffused in every 

 direction, and the blood tends to move backward as well as onward; all 

 movement backward, however, is prevented by the closure of the semi- 

 lunar valves, which takes place at the very commencement of the recoil 

 of the arterial walls. 



