ARTERIAL PRESSURE. 261 



ence in the caliber of the arteries at different periods of the 

 day. The diameter of the radial has been found very much 

 greater in the evening than in the morning, 1 producing, 

 naturally, a variation in the character of the pulse. We 

 learn from these physiological variations, how in disease, 

 when they become more considerable, they may give 

 important information with regard to the condition of the 

 system. 



Pressure of Blood in the Arteries. 



The reaction of the elastic walls of the arteries during the 

 intervals of the heart's action gives rise to a certain amount 

 of constant pressure, by which the blood is continually forced 

 toward the capillaries. The discharge of blood into the ca- 

 pillaries has a constant tendency to diminish this pressure ; 

 but the contractions of the left ventricle, by forcing repeated 

 charges of blood into the arteries, have a compensating ac- 

 tion. By the equilibrium between these two agencies, a 

 certain degree of tension is maintained in the arteries, which 

 is called the arterial pressure. 



The first experiments with regard to the extent of the 

 arterial pressure were made by Hales an English physiolo- 

 gist, more than a hundred years ago. 2 This observer, adapt- 

 ing a long glass tube to the artery of a living animal, ascer- 

 tained the height of the column of blood which could be 

 sustained by the arterial pressure. In some experiments on 

 the carotid of the horse, the blood mounted to the height of 

 from eight to ten feet. Hales was not fully acquainted with 

 the influences capable of modifying the arterial pressure, and 

 his estimates of the normal tension in these vessels were not 

 entirely correct. It is now ascertained that the pressure in 

 the arteries will sustain a column of about six feet of water, 

 or six inches of mercury, and is subject to considerable vari- 



1 MILNE-EDWARDS, op. cit., tome iv., p. 222. 



2 HALES, Statical Essays, London, 1733, vol. ii., HoemastaticJcs. 



