66 CIRCULATION OF THE BLOOD IN THE VESSELS. 



continually pressing their contents toward the periphery ; the dilatation of 

 the vessels with each systole of course admits an increased quantity of blood ; 

 and it has been shown that the same intermittent force exerted on an inelas- 

 tic tube will discharge a less quantity of liquid from openings of equal 

 caliber. 



Superadded, then, to the direct action of the heart, physiologists now rec- 

 ognize, as a cause influencing the flow of blood in the arteries, the resiliency of 

 the vessels, especially of those of large size, this force being derived originally 

 from the heart. Thus it will be seen that the arteries are constantly kept 

 distended with blood by the heart ; and by virtue of their elasticity and the 

 progressive increase in the capacity of this system as they branch, the power- 

 ful contractions of the central organ serve only to keep up an equable current 

 in the capillaries. The small vessels, by the action of their contractile walls, 

 regulate the local circulations. 



Locomotion of the Arteries and Production of the Pulse. With each 

 contraction of the heart, the arteries are increased in length and many of 

 them undergo a considerable locomotion. This may be readily observed in 

 vessels which are tortuous in their course, and is frequently very marked in 

 the temporal artery in old persons. The elongation may also be observed by 

 watching attentively the point where an artery bifurcates, as at the division 

 of the common carotid. It is simply the mechanical effect of sudden disten- 

 tion, which, while it increases the caliber of the vessel, causes an elongation 

 even more marked. 



The finger placed over an exposed artery or one which lies near the sur- 

 face experiences a sensation at every beat of the heart as though the vessel 

 were striking against it. This has long been observed and is called the pulse. 

 Ordinarily it is appreciated when the current of blood is subjected to a cer- 

 tain degree of obstruction, as in the radial, which can readily be compressed 

 against the bone. In an artery imbedded in soft parts which yield to press- 

 ure, the actual dilatation of the vessel being very slight, pulsation is felt with 

 difficulty, if at all. When obstruction of an artery is complete, as after tying 

 a vessel, the pulsation above the point of ligature is very marked and can be 

 readily appreciated by the eye. The explanation of this exaggeration of the 

 movement is the following : Normally, the blood passes freely through the 

 arteries and produces, in the smaller vessels, very little movement or dilata- 

 tion ; when, however, the current is obstructed, as by ligation or even com- 

 pression with the finger, the force of the heart is not sent through the vessel 

 to the periphery but is arrested and therefore becomes more marked and 

 easily appreciated. In vessels which have become undilatable and incom- 

 pressible from calcareous deposits, the pulse can not be felt. The character 

 of the pulse indicates, to a certain extent, the condition of the heart and 

 vessels. 



Under ordinary conditions, the pulse may be felt in all arteries that are 

 exposed to investigation ; and as it is due to the movement of the blood in 

 the vessels, the prime cause of its production is the contraction of the left 

 ventricle. The impulse given to the blood by the heart, however, is not felt 



