ON THE FUNCTIONS OF THE HEART AND ARTERIES* 113 



mere conjecture, and it would follow, that the pulse would 

 always become more and more full, as it became more dis- 

 tant f?om the heart ; of which we have nothing like evi- 

 dence : nor would a moderate contraction, even if this sup- 

 position were granted, produce any material effect. For 

 example, if the velocity of the contraction were only half 

 as great as that of the pulsation, which is the most favour- 

 able proportion, it would be necessary, taking sixteen feet 

 in a second for the velocity of the pulsation, that the sec- 

 tion of the arteries should be contracted-to about one half, 

 in order to produce, by their progressive contraction only, 

 the actual velocity of the blood in the aorta; one sixteenth 

 of the blood being carried, in this case, before the contrac- 

 tion: but if the contraction were only such, as to reduce 

 the section of the artery to t^^., which is probably more than 

 ever actually happens, the velocity produced would be only 

 about gV as much ; and if the contraction were only to ^^^, 

 which is a sufficient allowance for the smaller arteries, about 

 TffUT)T7 on^y of the actual velocity in the aorta could be pro* 

 duced in this manner, even upon a supposition much more 

 favourable to the muscular action of the arteries than the 

 actual circumstances. A small addition must be made to 

 the force required for producing the retrogade motion, on 

 account of the friction to be overcome, but the general rea- 

 soning is not affected by this correction. 



The contraction of the artery might also be supposed to or thdt the 



remain after each pulsation, so that the vessel should not <^""*'^*"'^" '■^' 

 , • Ti 1 -1 1 ^ ^ ' -1 , mains after 



be agam dilated until the next pulsation, or, in other words, each pulsatiesv 



a sppntaneous dilatatioii might be supposed to accompany 

 the pulsation, instead of a contraction : but such a dilata- 

 tion would be useless in promoting the progressive motion 

 of the blood, since a larger quantity of bipod, conveyed to 

 the smaller vessels, without aa increased tension, would be 

 ineffectual with respect to the resistances which are to be 

 overcome. It is possible indeed, that the muscular fibres 

 of those arteries in which the magnitude of the pulse is 

 sensible, hke the fibres of the heart, may be inactive, or 

 nearly so, during their dilatation ; and that they may con- 

 tract, after they have been once distended, with a force 

 which is in a certain degree permanent; the greater mo- 

 YoL. XXVII— Oct. 1810. I meatum 



