FO1RCE OF BLOOD IN ARTERIES. 



129 



FIG. 47. 



so considerable as to be perceptible to the finger, and this 

 double beat has received the technical name of "dicrotous" 

 pulse. As a diseased condition this has long been recognized, 

 but it is only since the invention of the sphygmograph that it 

 ha been found to belong in a certain degree to the normal 

 pulse also. 



Various theories have been framed to account for the dicro- 

 tism of the pulse. By some, it is supposed to be due to the 

 aortic valves, the sudden closure of which stops the incipient 

 regurgitation of blood into the ventricle, and causes a mo- 

 mentary rebound throughout the arterial system ; while Dr. 

 Sanderson considers it to be caused by a kind of rebound from 

 the periphery rather than from the central part of the circu- 

 lating apparatus. 



Force of the Blood in the Arteries. 



The force with which the ventricles act in their contraction, 

 and the reasons for believing it sufficient for the circulation of 

 the blood, have been already mentioned. Both calculation 

 and experiment have proved that very little of this force is 

 consumed in the arteries. Dr. 

 Thomas Young calculated that 

 the loss of force in overcoming 

 friction and other hindrances in 

 the arteries would be so slight, 

 that if one tube were introduced 

 into the aorta, and another into 

 any other artery, even into one as 

 fine as hair, the blood would rise 

 in the tube from the small vessel 

 to within two inches of the height 

 to which it would rise from the 

 large vessel. The correctness of 

 the calculation is established by 

 the experiments of Poiseuille, who 

 invented an instrument named a 

 hsemadynamometer, for estimating 

 the statical pressure exercised by 

 the blood upon the walls of the 

 arteries. It consists of a long glass 

 tube, bent so as to have a short 

 horizontal portion (b, Fig. 47), 

 a branch (a) descending at right 

 angles from it, and a long ascend- 

 ing branch (c). Mercury poured 

 into the ascending and descending 

 portions will necessarily have the 



