*9 
of the Heart and Arteries . 
the heart, may be inactive, or nearly so, during their dilatation, 
and that they may contract after they have been once dis- 
tended, with a force which is in a certain degree permanent ; 
the greater momentum of the blood, which accompanies the 
dilatation, enabling it to enter the minute arteries with equal 
ease, although assisted by a tension somewhat smaller : so that 
the same mean velocity may be sustained, as if the arteries 
were simply elastic, and a little smaller in diameter, with a 
very little less exertion of the heart. But the distribution of 
the blood could never be materially diversified by any opera- 
tion of this kind : for if any artery were for a moment dis- 
tended by such a variation, so as to exceed its natural diameter 
by one hundredth part only, a pressure would thence arise 
equivalent to that of a column about two inches high, which 
would, in spite of all resistances, immediately dissipate the 
blood with a considerable velocity, and completely prevent 
any local accumulation, unless the elastic powers of the vessel 
itself were diminished ; and this is, perhaps, the most impor- 
tant, as well as the best established inference from the doctrine 
that I have advanced. 
It appears that a mola has sometimes been found in the 
uterus, totally destitute of a heart, in which the blood must 
have circulated in its usual course through the veins and arte- 
ries : in this case it cannot be ascertained whether there was 
any alternate pulsation, or whether the blood was carried on 
in a uniform current, in the same manner as the sap of a ve- 
getable probably circulates. If there was a pulsation, it may 
have been maintained by a contraction of the artery, much 
more considerable, and slower in its progress than usual ; and 
with the assistance of a spontaneous dilatation ; the resistance 
D 2 
