284 THE CIRCULATION. 



same moment, yet those nearest the heart are distended suddenly 

 and rapidly, while for those at a distance, the distension takes place 

 more slowly and gradually. Thus the impulse given to the finger, 

 which marks the condition of maximum distension of the vessel, 

 occurs a little later at a distance from the heart, than in its imme- 

 diate proximity. 



This modification of the arterial pulse is produced in the follow- 

 ing way : 



The contraction of the left ventricle is a brusque, vigorous and 

 sudden motion. The charge of blood, thus driven into the arterial 

 system, meeting with a certain amount of resistance from the fluid 

 already filling the vessels, does not instantly displace and force 

 onward a quantity of blood equal to its own mass, but a large 

 proportion of its force is used in expanding the distensible walls 

 of the vessels. In the immediate neighborhood, therefore, the 

 expansion of the arteries is sudden and momentary, like the con- 

 traction of the heart itself. But this expansion requires for its 

 completion a certain expenditure, both of force and time ; so that 

 at a little distance farther on, the vessel is neither distended to the 

 same degree nor with the same rapidity. At the more distant 

 point, accordingly, the arterial impulse is less powerful and arrives 

 more slowly at its maximum. 



On the other hand, when the heart becomes relaxed, the artery 

 in its immediate neighborhood contracts upon the blood by its own 

 elasticity ; and as its contraction at this time meets with no other 

 resistance than that of the blood in the smaller vessels beyond, it 

 drives a portion of its own blood into them, and thus supplies these 

 vessels with a certain degree of distending force even in the inter- 

 vals of the heart's action. Thus the difference in size of the carotid 

 artery, at the two periods of the heart's contraction and its relaxa- 

 tion, is very marked ; for the degree of its distension is great when 

 the heart contracts, and its own reaction afterward empties it of 

 blood to a very considerable extent. But in the small branches of 

 the radial or ulnar artery, there is less distension at the time of the 

 cardiac contraction, because this force has been partly expended in 

 overcoming the elasticity of the larger vessels ; and there is less 

 emptying of the vessel afterward, because it is still kept partially 

 filled by the reaction of the aorta and its larger branches. In other 

 words, there is progressively less variation in size, at the periods of 

 distension and collapse, for the smaller and distant arteries than for 

 those which are larger and nearer the heart. 



