CIRCULATION OF THE BLOOD. 14 3 



expanding the vessel produces the -pulse in it, almost coincidently with 

 the systole of the left ventricle. As the force of the left ventricle, however, 

 is not expended in dilating the aorta only, the wave of blood passes on, 

 expanding the arteries as it goes, running as it were on the surface of the 

 more slowly traveling blood already contained in them, and producing the 

 pulse as it proceeds. 



The distension of each artery increases both its length and its diameter. 

 In their elongation, the arteries change their form, the straight ones be- 

 coming slightly curved, and those already curved becoming more so, but 

 they recover their previous form as well as their diameter when the ven- 

 tricular contraction ceases, and their elastic walls recoil. The increase of 

 their curves which accompanies the distension of arteries, and the succeed- 

 ing recoil, may be well seen in the prominent temporal artery of an old 

 person. In feeling the pulse, the finger cannot distinguish the sensation 

 produced by the dilatation from that produced by the elongation and 



FIG. 124. Diagram of the mode of action of the Sphygmograph. 



curving; that which it perceives most plainly, however, is the dilatation, 

 or return, more or less, to the cylindrical form, of the artery which has 

 been partially flattened by the finger. 



The pulse due to any given beat of the heart is not perceptible at 

 the same moment in all the arteries of the body. Thus, it can be felt 

 in the carotid a very short time before it is perceptible in the radial artery, 

 and in this vessel again before the dorsal artery of the foot. The delay 

 in the beat is in proportion to the distance of the artery from the heart, 

 but the difference in time between the beat of any two arteries never 

 exceeds probably -J to ^ of a second. 



A distinction must be carefully made between the passage of the ivave 

 along the arteries and the velocity of the stream (p. 165) of blood. Both 

 wave and current are present; but the rates at which they trawl are very 

 different; that of the wave 16 -5 to 33 feet per second (5 to 10 metres) 

 being twenty or thirty times as great as that of the current. 



The Sphygmograph. A great deal of light has been thrown on 

 what may be called the form of the pulse by the sphygmograph (Figs. 

 124 and 125). The principle on which the sphygmograph acts is very 



