328 CIRCULATION OF NUTRITIVE FLUID. 



tissue being thicker in the smaller branches, and the elastic tissue being 

 found in larger amount in the main trunks. 



583. It is chiefly to the simple physical property of Elasticity, thus 

 possessed by the Arterial tubes, that we owe the equalization of the 

 flow of blood ; and we may hence understand the reason, why the trunks 

 that are in nearest connexion with the heart, should be those most 

 endowed with it. If a forcing-pump were to inject water, by successive 

 strokes, into a system of tubes with perfectly unyielding walls, the flow 

 of fluid at the further extremities of these tubes would be as much 

 interrupted as its entrance into them. But if the pump be connected 

 with an air-vessel (as in the common fire engine), so that a part of the 

 force of each stroke is expended in compressing the air, the expansion 

 of this, during the interval between the successive strokes, produces a 

 continuous flow of water along the tubes. Or if the tubes themselves 

 were endowed with a certain degree of elasticity, which should allow 

 them to dilate near their commencement, so as to receive the new 

 charge of fluid, and which should occasion a continued pressure upon 

 the fluid during the interval of the stroke, the same equalizing effect 

 would be produced. This is precisely the case with the Arterial system ; 

 the intermittent jets, by which the blood is propelled from the heart, 

 are speedily converted into a continued stream ; so that, at even a mode- 

 rate distance from the heart, the only indication of its interrupted action 

 is presented by the greater or less rapidity of the flow ; and this gives 

 rise, when an artery is divided, to an alternate rise and fall of the jet 

 of blood, and, in the ordinary circulation, to >the phenomenon called the 

 pulse. This is due to an increase in the dimension of the arterial tube, 

 both in length and breadth, with each additional ingress of blood ; the 

 increase in length is the more considerable of the two effects, and causes 

 the artery to be somewhat lifted from its seat. During the intervals, a 

 quantity of blood corresponding to that which had entered, escapes 

 by the further extremity of the tube; and thus the artery is enabled 

 to contract to its previous dimensions, x and to return to its bed. We 

 may compare the pulse, therefore, to a wave, which commences in the 

 heart, and travels onwards through the arterial system. 



584. In the large arteries near the heart, the pulsation is always 

 precisely synchronous with the ventricular systole ; but it takes place 

 somewhat later in the arteries at a distance from the heart ; the time 

 required for the transmission of the wave being proportioned to the 

 degree in which the walls of the arteries yield to it. If they were quite 

 rigid, the egress at one extremity must take place at the precise moment, 

 that the fluid is forced into the other. On the other hand, if the walls 

 be too easily distensible, they yield to the propelling force in such a 

 degree that it is entirely expended upon them ; and the fluid is not 

 moved onward at all or but very slowly. In the healthy state of the 

 arterial walls, they should contract upon their contents with sufficient 

 force to equalize the flow of blood, and to prevent the pulse-wave from 

 occupying more than one-sixth or one-seventh of a second, in its propa- 

 gation to the remotest arteries of the system ; and the pulse should be 

 full, producing a prolonged but gentle elevation beneath the finger, and 

 capable of resisting moderate pressure. This condition is dependent 



