18 Dr. Young’s Lecture on the Functions 
example, if the velocity of the contraction were only half as 
great as that of the pulsation, which is the most favourable 
proportion, it would be necessary, taking sixteen feet in a se- 
cond for the velocity of the pulsation, that the section of the 
arteries should be contracted to about one half, in order to 
produce, by their progressive contraction only, the actual ve- 
locity of the blood in the aorta ; one sixteenth of the blood 
being carried, in this case, before the contraction: but if the 
contraction were only such, as to reduce the section of the ar- 
tery to T ^, which is probably more than ever actually happens, 
the velocity produced would be only about ~ as much ; and 
if the contraction were only to which is a sufficient allow- 
ance for the smaller arteries, about To - * 0 -- only of the actual 
velocity in the aorta could be produced in this manner, even 
upon a supposition much more favourable to the muscular ac- 
tion of the arteries than the actual circumstances. A small 
addition must be made to the force required for producing the 
retrograde motion, on account of the friction to be overcome, 
but the general reasoning is not affected by this correction. 
The contraction of the artery might also be supposed to 
remain after each pulsation, so that the vessel should not 
be again dilated until the next pulsation, or, in other words, 
a spontaneous dilatation might be supposed to accompany the 
pulsation, instead of a contraction : but such a dilatation would 
be useless in promoting the progressive motion of the blood, 
since a larger quantity of blood, conveyed to the smaller ves- 
sels, without an 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, like the fibres of 
