THE ARTERIAL CIRCULATION. 333 



the immediate neighborhood, therefore, the expansion of the arteries is 

 sudden and momentary, like the contraction 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 

 distended neither to the same degree nor with the same rapidity. At 

 the more distant point 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 reacts upon the blood by its own elasticity ; 

 and as it 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 to these vessels a certain degree of distending force 

 even in the intervals of the heart's action. Thus the difference in size 

 of the carotid artery, at the two periods of the heart's contraction and 

 relaxation, is ve^ marked ; for the degree of its distension is great 

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

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

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

 impulse, 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. 



These facts have been illustrated by Marey, 1 by attaching to the pipe 

 of a small forcing pump, worked by alternate strokes of the piston, a 

 long elastic tube open at its farther extremity. At different points 

 upon this tube are placed small movable levers, which are raised by the 

 distension of the tube whenever water is driven into it by the forcing 

 pump. Each lever carries upon its extremity a small pencil, which 

 marks upon a strip of paper, moving with uniform rapidity, the lines 

 produced by its alternate elevation and depression. By these curves 

 both the extent and rapidity of distension of different parts of the elastic 

 tube are accurately registered. The curves thus produced are as follows : 



Fig. 111. 



CURVES OF PULSATION IN AN ELASTIC TUBE. 1. Near the distending force 

 2. At a distance from it. 3. Still farther removed. 



From these experiments it is shown that the distension produced by 

 the stroke of the forcing pump begins at the same moment throughout 



1 Journal de la Physiologie. Paris, Avril, 1859 



