292 FUNCTIONS OF NUTRITION. 



which increases at each cardiac impulse, and diminishes during relaxa- 

 tion. 



Equalization of the Blood-current in the Arterial System. Since 

 the distensible and elastic properties of the arterial walls make the flow 

 of blood more continuous than it would otherwise be, this effect 

 increases as the blood moves through the arterial system. A part 

 of the force of each pulsation is absorbed, for the time being, in 

 the distension of the artery, and is again returned in an impulse to 

 the blood at the following- interval, by the reaction of the vessel. The 

 farther from the heart the blood recedes, the greater becomes the 

 influence of the intervening arteries ; and thus the remittent or pulsating 

 character of the arterial current, which is strongly pronounced in the 

 vicinity of the heart, becomes gradually diminished during its passage 

 through the vessels, until in the smaller arteries it is hardly per- 

 ceptible. 



The influence of an elastic medium, in equalizing the flow of an in- 

 terrupted current, may be shown by injecting water from a force-pump 

 alternately through two tubes, one of India-rubber, the other of metal. 

 Whatever be the length of the metallic tube, the water will be 

 delivered from its extremity in distinct jets, corresponding with the 

 strokes of the piston : but when it is replaced by an elastic tube of 

 sufficient length, the separate impulses are merged into each other, 

 and the water is discharged in a continuous stream. 



The elasticity of the arteries never completely neutralizes the effect 

 of the cardiac contractions, since a pulsation can be seen in the flow 

 of blood in even the smallest arteries, when examined under the mi- 

 croscope ; but it diminishes in degree from the heart outward, and 

 the current becomes nearly continuous at the confines of the capillary 

 system. 



The Arterial Pressure. The arterial circulation, as shown by the 

 above facts, is the combined result of two different forces ; namely, the 

 contraction of the heart, by which the blood is propelled in successive 

 impulses, and the elasticity of the arteries, by which it is subjected to 

 a continuous pressure. 



If one of the larger or medium-sized arteries be divided, in the living 

 animal, and a glass tube of the same diameter fixed in its orifice 

 in the vertical position, the blood will rise in the tube to a height 

 of five and a half or six feet, and, until coagulation occurs, will continue 

 to oscillate about this level. The column of fluid thus supported in- 

 dicates the pressure to which the blood is subjected within the vessels. 

 This force, due to the reaction of the arterial system, is known as the 

 arterial pressure. 



The arterial pressure is best measured by connecting an open artery r 

 by means of a flexible tube, with a small reservoir of mercury, provided 

 with a narrow upright graduated tube, open at its upper extremity. 

 When the mercury in the reservoir is exposed to the pressure of the 

 blood, it rises in the upright tube to a corresponding level. 



