THE PULSE. 161 



the rising of the wave the arteries nearest the heart are 

 dilated and elongated. They do not at once recoil, but 

 .continue to be distended so long as blood is entering them 

 from the ventricle. The wave at the head of the more 

 rapid stream of blood runs on, propelled and maintained 

 in its velocity by the continuous contraction of the ventricle : 

 and it thus dilates in succession every portion of the 

 arterial system, and produces the pulse in all. At length, 

 the whole arterial system (wherein a pulse can be felt) is 

 dilated; and at this time, when the wave we have sup* 

 posed has reached all the smaller arteries, the entire 

 system may be said to be simultaneously dilated ; then it 

 begins to contract, and the contractions of its several parts 

 ensue in the same succession as the dilatations, commencing 

 at the heart. The contraction of the first portion produces 

 the closure of the valves and the second sound of the 

 heart ; and both it and the progressive contractions of all 

 the more distant parts maintain, as already said, that 

 pressure on the blood during the inaction of the ventricle, 

 by which the stream of the arterial blood is sustained 

 between the jets, and is finally equalized by the time it 

 reaches the capillaries. 



It may seem an objection to this theory, that it would 

 probably require a larger quantity of blood to dilate all 

 the arteries than can be discharged by the ventricle at each 

 contraction. But the quantity necessary for such a pur- 

 pose is less than might be supposed. Injections of the 

 arteries prove that, including all down to those of about 

 one-eighth of a line in diameter, they do not contain on. 

 an average more than one and a half pints of fluid, even 

 when distended. There can be no doubt, therefore, that the 

 three or four ounces which the ventricle is supposed to dis- 

 charge at each contraction, being added to that which already 

 fills the arteries, would be sufficient to distend them all. 



Besides this wave-like movement, however, it must bo 

 remembered that, as before stated, the pulse legins in 



M 



