brain and nervous system seems well adapted to distribute it over every
part of the body.

For the medullary substance of the brain not only occupies the cavities of
the head and spine, but passes along the innumerable ramifications of the
nerves to the various muscles and organs of sense. In these it lays aside
its coverings, and is intermixed with the slender fibres, which constitute
those muscles and organs of sense. Thus all these distant ramifications of
the sensorium are united at one of their extremities, that is, in the head
and spine; and thus these central parts of the sensorium constitute a
communication between all the organs of sense and muscles.

3. A _nerve_ is a continuation of the medullary substance of the brain from
the head or spine towards the other parts of the body, wrapped in its
proper membrane.

4. The _muscular fibres_ are moving organs intermixed with that medullary
substance, which is continued along the nerves, as mentioned above. They
are indued with the power of contraction, and are again elongated either by
antagonist muscles, by circulating fluids, or by elastic ligaments. So the
muscles on one side of the forearm bend the fingers by means of their
tendons, and those on the other side of the fore-arm extend them again. The
arteries are distended by the circulating blood; and in the necks of
quadrupeds there is a strong elastic ligament, which assists the muscles,
which elevate the head, to keep it in its horizontal position, and to raise
it after it has been depressed.

5. The _immediate organs of sense_ consist in like manner of moving fibres
enveloped in the medullary substance above mentioned; and are erroneously
supposed to be simply an expansion of the nervous medulla, as the retina of
the eye, and the rete mucosum of the skin, which are the immediate organs
of vision, and of touch. Hence when we speak of the contractions of the
fibrous parts of the body, we shall mean both the contractions of the
muscles, and those of the immediate organs of sense. These _fibrous
motions_ are thus distinguished from the _sensorial motions_ above
mentioned.

6. The _external organs_ of sense are the coverings of the immediate organs
of sense, and are mechanically adapted for the reception or transmission of
peculiar bodies, or of their qualities, as the cornea and humours of the
eye, the tympanum of the ear, the cuticle of the finders and tongue.

7. The word _idea_ has various meanings in the