ts, and those parts of other bones which are covered with
cartilages, are surrounded by a fine membrane, which on the skull
is called pericranium, but in other parts periosteum. This
membrane serves for the muscles to slide easily upon, and to
hinder them from being lacerated by the hardness and roughness of
the bones.

But though the apparatus which I have been describing is admirably
contrived for the performance of motion; it would continue for
ever inactive, if not animated by the nervous system.

The brain is the seat of the intelligent principle: from this
organ, white, soft, and medullary threads, called nerves, are sent
off to different parts of the body: some of them proceed
immediately from the brain to their destined places, while the
greater number, united together, perforate the skull, and enter
the cavity of the backbone, forming what we call the spinal
marrow, which may be regarded as a continuation of the brain.
Portions of the spinal marrow pass through different apertures to
all parts of the body.

We are not conscious of the impression of external objects on our
body, unless there be a free communication of nerves, between the
place where the impression is made and the brain. If a nerve be
divided, or have a ligature put round it, sensation is intercepted.

There is perhaps only one sense which is common to all classes of
animals, and which exists over every part of the surface of the
body; I mean the sense of touch. The seat of this sense is in the
extremities of the nerves distributed over the skin; and by means
of it we ascertain the resistance of bodies, their figure, and
their temperature.

The other senses have been thought to be only more refined
modifications of the sense of touch; and the organs of each are
placed near the brain on the external surface of the head. The
sense of sight, for instance, is seated in the eye; the hearing in
the ear; the smell in the internal membrane of the nose; and the
taste in the tongue.

The light; the pulses, or vibrations of the air; the effluvia
floating in the atmosphere; saline particles, or particles which
are soluble in water or saliva, are the substances which act upon
these four senses; and the organs which transmit their action to
the nerves, are admirably adapted to the respective nature of
each. The eye presents to the light a succession of transparent
lenses to refract its rays; the ear opposes to the air membranes,
fluids, and bones, we