shed. The contrary, however, will take place on a
horizontal swing, as I have frequently observed, both on myself and
others; for the centrifugal force in this case will propel the blood
from the extremities towards the head.

It has been already observed, that the pulsations of the artery which
we feel at the wrist, are occasioned by its alternate dilatations and
contractions, which vary according to the strength and regularity of
the circulation, which is liable to be affected by the smallest
changes in the state of health. Hence physicians make use of the
pulse as a criterion whereby to judge of the health of the body. And we
may observe that there are few more certain characteristics of the
state of the body than the pulse; yet the conclusions that have been
drawn from it have often been erroneous; and this has arisen from
trusting to observation without the aid of reason.

That we may better understand the phenomena of the pulse, I shall lay
down the following postulata. 1st. It is now generally believed, that
every part of the arterial system is endowed with irritability, or a
power of contracting on the application of a stimulus, and that the
blood acting on this contractibility, if the term may be allowed,
causes contraction; and that the alternate relaxation and contraction
gives the phenomenon pulsation. 2d. The greater the action of the
stimulus of the blood, the greater will be the contraction, that is,
the nearer will the sides of the artery approach towards the axis.
3d. That the velocity with which a muscular fibre, in a state of
debility, contracts, is at least equal to that with which a fibre in
a state of strength contracts, is a fact generally allowed by
physiologists.

We shall afterwards see, that a deficient action of stimulus on the
vessels may arise, either directly from diminishing the quantity of
blood contained in them, or indirectly, from the application of too
great a stimulant power, which has diminished the capability of
contracting inherent in the vessels.

From these postulata, it will be evident, that the greater the action
of the arteries, that is, the more powerful their contraction, the
longer will be the intervals between the pulsations.

For the velocity being at least equal in debility and in strength,
the times between the pulsations will be proportioned to the approach
of the sides of the artery towards its axis: but the approach of the
sides towards the axis is greater wh