the intervals between those contractions being less also, accounts for the
quick pulse in fevers with debility, and in dying animals. The shortness of
the intervals between one contraction and another in weak constitutions, is
probably owing to the general deficiency of the quantity of the spirit of
animation, and that therefore there is a less quantity of it to be received
at each interval of the activity of the fibres. Hence in repeated motions,
as of the fingers in performing on the harpsichord, it would at first sight
appear, that swiftness and strength were incompatible; nevertheless the
single contraction of a muscle is performed with greater velocity as well
as with greater force by vigorous constitutions, as in throwing a javelin.

There is however another circumstance, which may often contribute to cause
the quickness of the pulse in nervous fevers, as in animals bleeding to
death in the slaughter-house; which is the deficient quantity of blood;
whence the heart is but half distended, and in consequence sooner
contracts. See Sect. XXXII. 2. 1.

For we must not confound frequency of repetition with quickness of motion,
or the number of pulsations with the velocity, with which the fibres, which
constitute the coats of the arteries, contract themselves. For where the
frequency of the pulsations is but seventy-five in a minute, as in health;
the contracting fibres, which constitute the sides of the arteries, may
move through a greater space in a given time, than where the frequency of
pulsation is one hundred and fifty in a minute, as in some fevers with
great debility. For if in those fevers the arteries do not expand
themselves in their diastole to more than half the usual diameter of their
diastole in health, the fibres which constitute their coats, will move
through a less space in a minute than in health, though they make two
pulsations for one.