THE ARTERIAL CIRCULATION. 337 



be everywhere intermittent ; and it would be delivered from an orifice 

 situated at any point, in perfectly interrupted jets. But the arteries 

 are yielding and elastic ; and this elasticity moderates the force of the 

 separate arterial pulsations, and partially fuses them with each other. 

 The effect of this is to produce, in the larger and medium-sized arteries, 

 a movement of the blood which is increased in rapidity and volume at 

 each cardiac impulse, and diminished in the interval of relaxation. 



Equalization of the Blood-current in the peripheral parts of the 

 Arterial System. It has already been shown that the distensible and 

 elastic properties of the arterial walls have the effect of making the flow 

 of blood more continuous than it would be if subjected only to the 

 intermitting action of the heart. A part of the force of each cardiac 

 pulsation is absorbed for the time in the distension of the artery ; and 

 this force is again returned in the form of an impulse to the blood at 

 the following interval, by the elastic 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, like the labials, it is hardly percep- 

 tible to the unaided eye. 



The physical influence of an elastic medium, in equalizing the force 

 of an interrupted current, may be shown by forcing water from a 

 syringe alternately through two tubes, one of India rubber, the other of 

 glass or metal. Whatever be the length of the inelastic tube, the water 

 thrown into one extremity will be delivered from the other in distinct 

 jets, corresponding with the strokes of the piston : but if the metallic 

 tube be replaced by one of India rubber of sufficient length, the elas- 

 ticity of this substance merges the separate impulses into each other, 

 and the water is discharged from the farther extremity in a continuous 

 stream. 



The elasticity of the arteries never entirely equalizes the force of the 

 separate pulsations, since a pulsating character can be seen in the flow 

 of the blood through even the smallest arteries, if examined under the 

 microscope ; but this pulsating character diminishes from the heart out- 

 ward, and the current becomes much more continuous in the smaller 

 vessels than in the larger arteries or in those of medium size. 



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

 above facts, is not an entirely simple phenomenon, but is the combined 

 result of two different physical forces. It is due, first, to the intermit- 

 ting action of the heart, by which the blood is driven in successive im- 

 pulses from within outward ; and, secondly, to the elasticity of the entire 

 arterial system, by which it is subjected to a continuous pressure. 



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

 living animal, and a glass tube of the same diameter securely fixed in 

 its open orifice and held in the vertical position, the blood will at once 

 rise in the tube to a height of five and a half or six feet, and will con- 



