286 THE CIRCULATION. 



the flow of the blood through it is seen to be entirely intermittent. 

 A strong jet takes place at each ventricular contraction, and at each- 

 relaxation the flow is completely interrupted. If the puncture be 

 made, however, in any of the large arteries near the heart, the flow 

 of blood through the orifice is no longer intermittent, but is con- 

 tinuous ; only it is very much stronger at the time of ventricular 

 contraction, and diminishes, though it does not entirely cease, at 

 the time of relaxation. If the blood were driven through a series 

 of perfectly rigid and unyielding tubes, its flow would be every- 

 where intermittent ; and it would be delivered from an orifice situ- 

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

 are yielding and elastic; and this elasticity, as we have already 

 explained, moderates the force of the separate arterial pulsations, 

 and gradually fuses them with each other. The interrupted or 

 pulsating character of the arterial current, therefore, which is 

 strongly pronounced in the immediate vicinity of the heart, becomes 

 gradually lost and equalized, during its passage through the vessels, 

 until in the smallest arteries it is nearly imperceptible. 



The same effect of an elastic medium in equalizing the force of 

 an interrupted current may be shown by fitting to the end of a 

 common syringe a long glass or metallic tube. Whatever be the 

 length of the inelastic tubing, the water which is thrown into one 

 extremity of it by the syringe will be delivered from the other end 

 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 elasticity of this substance merges the force of the sepa- 

 rate impulses into each other, and the water is driven out from the 

 farther extremity in a continuous stream. 



The elasticity of the arteries, however, never entirely equalizes 

 the force of the separate cardiac pulsations, since a pulsating cha- 

 racter can be seen in the flow of the blood through even the smallest 

 arteries, under the microscope ; but this pulsating character dimi- 

 nishes very considerably from the heart outward, and the current 

 becomes much more continuous in the smaller vessels than in the 

 larger. 



The primary cause, therefore, of the motion of the blood in the 

 arteries is the contraction of the ventricles, which, by driving out 

 the blood in interrupted impulses, distends at every stroke the 

 whole arterial system. But the arterial pulse is not exactly syn- 

 chronous everywhere with the beat of the heart ; since a certain 

 amount of time is required to propagate the blood-wave from the 



