164 



THE CIRCULATION. 



Force of the Blood in the Arteries. 



The force with which the ventricles act in their con- 

 traction, 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 

 Fig. 47.* 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 cor- 

 rectness of the calculation is estab- 

 lished by the experiments of Poiseu- 

 ille, who invented an instrument 

 named a hsemadynamometer, for es- 

 timating the statical pressure exer- 

 cised 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 (fig. 47), a branch 

 (2) descending at right angles from it, 

 and a long ascending branch (3). 

 Mercury poured into the ascending 

 and descending portions, will necessarily have the same 

 level in both branches, and in a vertical position the 

 height of its column must be the same in both. If, now, 

 the blood is made to flow from an artery, through the 

 horizontal portion of the tube (which should contain a 

 solution of carbonate of potash to prevent coagulation) 

 into the descending branch, it will exert on the mercury a 



