266 CIRCULATION. 



constriction in the tube offers such an obstacle to the rise of 

 the mercury that the intermittent action of the heart is not 

 felt, the mercury rising slowly to a certain level, which is con- 

 stant, and varies only with the constant pressure in the vessels. 



"We have only an approximative idea of the average press- 

 ure in the arterial system in the human subject, deduced from 

 experiments on animals. It has already been stated to be 

 equal to about six feet of water, or six inches of mercury. 



The most interesting questions connected with this sub- 

 ject are : the comparative pressure in different parts of the 

 arterial system, the influences which modify the arterial press- 

 ure, and its influence on the pulse. These points have all 

 been pretty fully investigated by experiments on animals, and 

 on systems of elastic tubes arranged to represent the vessels. 



Pressure in Different Parts of the Arterial System. 

 The experiments of Hales, Poiseuille, Bernard, and others, 

 seem to show that the constant arterial pressure does not vary 

 in arteries of different sizes. These physiologists have ex- 

 perimented particularly on the carotid and crural, and have 

 found the pressure in these two vessels about the same. 

 From their experiments, they conclude that the force is 

 equal in all parts of the arterial system. The experiments of 

 Yolkmann, however, have shown that this conclusion has been 

 too hasty. With the registering apparatus of Ludwig, he has 

 taken the pressure in the carotid and metatarsal arteries, and 

 has always found a considerable difference in favor of the 

 former. 1 In an experiment on a dog, he found the pressure 



1 For comparing the pressure in different vessels and in different animals, 

 Bernard has devised an instrument which he calls the differential hemodynamo- 

 rneter. It consists of a graduated U tube so arranged that both arms may be 

 simultaneously connected with separate vessels. If the pressure be equal in the 

 two vessels with which it is connected, the level of the mercury will not be affect- 

 ed ; but an inequality of pressure will be marked by a depression of the mercury 

 in the arm corresponding to the vessel in which the pressure is the more power- 

 ful. With this instrument, Bernard assumes to have demonstrated that the con- 

 stant pressure is equal in all parts cf the arterial system, the force of the heart, 



