270 



CIRCULATION. 



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 

 demonstrated that an increase of the arterial pressure dimin- 

 ishes the amplitude of the pulsations, as indicated by the 

 sphygmograph, and that the amplitude is very great when 

 the pressure is slight. 



It is also true, as a general rule, that the force of the heart, 

 as indicated by the cardiometer, bears an inverse ratio to the 

 frequency of its pulsations. 



Summary. The arterial pressure, due to the distention 

 of the arteries, and the reaction of their elastic walls contin- 

 ually forcing the blood toward the capillaries, is equal to 

 about six feet of water or six inches of mercury. It is in- 

 creased by any thing which favors the flow of blood into the 

 great vessels, like the expiratory act, or by any thing which 

 obstructs the flow from the arterioles into the capillaries, 

 like muscular effort, contraction of the muscular coat of the 

 smallest arteries, or non-aeration of the blood. It is dimin- 

 ished by any considerable diminution in the quantity of the 

 circulating fliiid, or .by any thing which facilitates the passage 

 of blood through the capillaries. 



Rapidity of the Current of Blood in the Arteries. 



Though this is not a question of great physiological im- 

 portance, it is a point of some interest. It has long engaged 

 the attention of physiologists, and has lately been made the 

 subject of some curious and ingenious experiments. Passing 

 over the speculations and calculations from imperfect physi- 

 cal data of the older physiologists, which led to no definite 

 results, we find the first experiments on this subject made by 

 Yolkmann, with an instrument called the hemodromometer. 

 This apparatus consists of a U tube, graduated, and so ar- 



