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 writers of metaphysic: it is
here used simply for those notions of external things, which our organs of
sense bring us acquainted with originally; and is defined a contraction, or
motion, or configuration, of the fibres, which constitute the immediate
organ of sense; which will be explained at large in another part of the
work. Synonymous with the word idea, we shall sometimes use the words
_sensual motion_ in contradistinction to _muscular motion_.