m trunks, whose capacities are smaller than the sums of the
capacities of the branches, hence the velocity of the blood in the
veins will increase as it approaches the heart.

Another retarding cause may be mentioned, namely, gravity, which acts
more on the venous than the arterial system. The effects of gravity
on the veins may be exemplified, by a ring being pulled off the
finger with ease when the hand is elevated; also by the swellings of
the feet that occur in relaxed habits, which swellings increase
towards night, and subside in the morning, after the body has been in
a horizontal posture for some hours.

In weak persons, the frequency of the pulse is increased by an erect
posture, which may probably depend on gravity; as we know, from the
observations of Macdonald and others, that an erect posture will make
a difference of 15 or 20 beats in a minute. The experiments alluded
to, were made by gently raising a person fastened to a board, where
there being no muscular exertion, respiration would not be increased;
so that the whole effect was probably owing to gravity accelerating
the column of arterial blood.

The inverted posture produces a still more remarkable effect in
accelerating the pulse, than the erect, for it sometimes causes it to
beat 10 or 12 times more in the former case than in the latter.

While we are on this subject, it may not be improper to take notice
of the effects of swinging on the circulation, which have been found
by Dr. Carmichael Smyth, and others, to diminish the strength and
velocity to such a degree, as to bring on fainting. These effects
have never been satisfactorily accounted for; but they would seem to
admit of an easy explanation on mechanical principles: they are
undoubtedly owing, at least in a great measure, to the centrifugal
force acquired by the blood.

By a centrifugal force, I mean, the tendency which revolving bodies
have to fly off from the centre, which arises from their tendency to
move in a straight line, agreeably to the laws of motion. Hence a
tumbler of water may be whirled in a circle vertically without
spilling it; the centrifugal force pushing the water against the
bottom of the tumbler. In the same manner when the human body is made
to revolve vertically in the arch of a circle, this centrifugal force
will propel the blood from the head and heart towards he extremities;
hence the circulation of the blood will be weakened, and the energy of
the brain dimini