PRESSURE OF BLOOD IN THE ARTERIES. 71 



more wiry character. As a rule, prolonged contraction of the arteries is fol- 

 lowed by relaxation, as is seen in the full pulse and glow of the surface which 

 accompany reaction after exposure to cold. It has been found, also, that 

 there is a considerable difference 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, producing, naturally, a variation 

 in the character of the pulse. 



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 degree of pressure, by which the 

 blood is continually forced toward the capillaries. The discharge of blood 

 into the capillaries 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 action. By the equilibrium between 

 these two agencies, a certain 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 physiologist, more than a hundred years 

 ago. This observer, adapting a long glass tube to the artery of a living ani- 

 mal, ascertained 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 eight to ten feet (243 to 304 centi- 

 metres). 



If a large artery, like the carotid, be exposed in a living animal, and a 

 metallic point, connected with a vertical tube of smaller caliber and seven or 

 eight feet (213 or 243 centimetres) long by a bit of elastic tubing, be secured 

 in the vessel, the blood will rise to the height of about six feet (183 centi- 

 metres) and remain at this point almost stationary, indicating, by a slight 

 pulsatile movement, the action of the heart. On carefully watching the level 

 in the tube, in addition to the rapid oscillation coincident with the pulse, 

 another oscillation will be observed, which is less frequent and which corre- 

 sponds with the movements of respiration. The pressure, as indicated by an 

 elevation of the fluid, is slightly increased during expiration and diminished 

 during inspiration. In such experiments, it is necessary to fill part of the 

 tube, or whatever apparatus be used, with a solution of sodium carbonate, in 

 order to prevent coagulation of the blood as it passes out of the vessels. 



The experiment with the long tube gives, perhaps, the best general idea 

 of the arterial pressure, which will be found to vary between five and a half 

 and six feet of blood (170 and 183 centimetres), or a few inches more of 

 water. The oscillations produced by the contractions of the heart are not 

 very marked, on account of the great friction in so long a tube ; but this is 

 favorable to the study of the constant pressure. It has been found that the 

 estimates above given do not vary very much in animals of different sizes. 

 Bernard found the pressure in the carotid of a horse but little more than in 

 the dog or rabbit. In the larger animals, it is the force of the heart which 



