en the arteries are in a state of
vigour than when debilitated; consequently the intervals between the
pulsations will be greater when the arteries are in a state of vigour
than when debilitated.

Hence it is evident, that a frequency of pulse must generally
indicate a diminished action or debility; while a moderate slowness
indicates a vigorous or just action.

Hence likewise the opinion of increased action, which has been
supposed to take place in fevers, because a frequent pulse was
observed, must be false, because the frequency arises from a directly
opposite state, and indicates a diminished action of the vascular
system.

In a sound and adult man the frequency of the pulse is about seventy
beats in a minute; and in an infant, within the first five or six
months, the pulse is seldom less than one hundred and twenty, and
diminishes in frequency as the child grows older. But though seventy
beats in the minute may be taken as a general standard; yet in
persons of irritable constitutions the frequency is greater than
this, and many, who are in the prime of life, have the pulse only
between fifty and sixty.

It is generally observed, that the pulse is slower in the morning,
that it increases in frequency till noon, after dinner it again
becomes slow, and in the evening its frequency returns, which
increases till midnight.

These phenomena may be rationally explained on the principles just
laid down. When we rise in the morning, the contractibility being
abundant, the stimulus of the blood produces a greater effect, the
pulse becomes slow, and the contractions strong; it becomes more
frequent, however, till dinner time, from a diminished
contractibility; after dinner, from the addition of the stimulus of
food and chyle, it again decreases in frequency, and becomes slow
till the evening, when its frequency returns, because the
contractibility becomes exhausted: and this frequency continues till
the vital power have been recruited by sleep.

By the same principles it is easy to explain the quickness of the
pulse in infancy, its gradual decrease till maturity, its slowness
and strength during the meridian of life, and the return of its
frequency during the decline.

Having now described the phenomena of the circulation, it will be
proper to examine the changes produced by this function on the blood;
and, in the first place, it may be observed, that the blood which
returns by the vena cava to the heart, is of a da