60 CIRCULATION OF THE BLOOD 



Influence of Muscular Action, etc. Muscular effort considerably 

 increases the arterial pressure. This is due to two causes. In the 

 first place, the chest is compressed, and this favors the flow of blood 

 into the great vessels. In the second place, muscular exertion pro- 

 duces a certain degree of obstruction to the discharge of blood from the 

 arteries into the capillaries. Experiments on the inferior animals 

 show a great increase in pressure in the struggles that occur during 

 severe operations. It has been shown that stimulation of the sympa- 

 thetic in the neck and of certain of the cerebro-spinal nerves increases 

 the arterial pressure, probably from an influence on the muscular coats 

 of some of the arteries, causing them to contract and thereby dimin- 

 ishing the total capacity of the arterial system. 



Effects of Hemorrhage, etc. Diminution in the quantity of blood 

 has a remarkable effect on arterial pressure. If, in connecting the 

 instrument with the arteries, even one or two jets of blood are allowed 

 to escape,, the pressure will be found diminished perhaps one-half or 

 even more. It is hardly necessary to discuss the mechanism of the 

 effect of the loss of blood on the tension in the vessels, but it is re- 

 markable how soon the pressure in the arteries regains the normal 

 standard after it has been lowered by hemorrhage. As the pressure 

 depends largely on the quantity of blood, so soon as the vessels absorb 

 serosities in sufficient quantity to repair the loss, the pressure is in- 

 creased. This takes place in a short time, if the loss of blood is not 

 too great. 



Experiments on arterial pressure with the cardiometer have verified 

 the fact mentioned in treating of the form of the pulse ; namely, that 

 the pressure in the vessels bears an inverse ratio to the distention pro- 

 duced by the contractions of the heart. In the cardiometer, the mean 

 height of the mercury indicates the constant, or arterial pressure; and 

 the oscillations, the distention produced by the heart. It is found that 

 when the pressure is great, the extent of oscillation is small, and vice 

 versa. It will be remembered that the researches of Marey demon- 

 strated that an increase in arterial pressure diminishes the amplitude 

 of the pulsations, as indicated by the sphygmograph, and that the 

 amplitude is great when the pressure is slight. It is also true, as a 

 general rule, that the force of the heart, as indicated by the cardi- 

 ometer, bears an inverse ratio to the frequency of its pulsations. 

 I An instrument called the sphygmomanometer has been devised for 

 measuring the arterial pressure in the human subject. This is for use 

 chiefly in clinical work and its description here would be out of place. 

 The principle on which it is constructed is to measure by means of 

 a column of mercury the pressure required to arrest the pulse at the 



