ion of
the air does from the currents of it. Thus are the different organs of our
bodies stimulated by four different mechanic properties of the external
world: the sense of touch by the pressure of solid bodies so as to
distinguish their figure; the muscular system by the distention, which they
occasion; the internal surface of the arteries, by the momentum of their
moving particles; and the auditory nerves, by the vibration of them: and
these four mechanic properties are as different from each other as the
various chemical ones, which are adapted to the numerous glands, and to the
other organs of sense.

2. The momentum of the progressive particles of blood is compounded of
their velocity and their quantity of matter: hence whatever circumstances
diminish either of these without proportionally increasing the other, and
without superadding either of the general stimuli of heat or distention,
will tend to produce a quiescence of the arterial system, and from thence
of all the other irritative motions, which are connected with it.

Hence in all those constitutions or diseases where the blood contains a
greater proportion of serum, which is the lightest part of its composition,
the pulsations of the arteries are weaker, as in nervous fevers, chlorosis,
and hysteric complaints; for in these cases the momentum of the progressive
particles of blood is less: and hence, where the denser parts of its
composition abound, as the red part of it, or the coagulable lymph, the
arterial pulsations are stronger; as in those of robust health, and in
inflammatory diseases.

That this stimulus of the momentum of the particles of the circulating
fluid is of the greatest consequence to the arterial action, appears from
the experiment of injecting air into the blood vessels, which seems to
destroy animal life from the want of this stimulus of momentum; for the
distention of the arteries is not diminished by it, it possesses no
corrosive acrimony, and is less liable to repass the valves than the blood
itself; since air-valves in all machinery require much less accuracy of
construction than those which are opposed to water.

3. One method of increasing the velocity of the blood, and in consequence
the momentum of its particles, is by the exercise of the body, or by the
friction of its surface: so, on the contrary, too great indolence
contributes to decrease this stimulus of the momentum of the particles of
the circulating blood, and thus te