It is the extreme mobility of which man is capable, owing to his
peculiar organization, that distinguishes him from other beings that are
called insensible or inanimate; the different degrees of this mobility,
of which the individuals of his species are susceptible, discriminate
them from each other; make that incredible variety, that infinity of
difference which is to be found, as well in their corporeal faculties,
as in those which are mental or intellectual. From this mobility, more
or less remarkable in each human being, results wit, sensibility,
imagination, taste, &c.: for the present, however, let us follow the
operation of the senses; let us examine in what manner they are acted
upon, and are modified by exterior objects:--we will afterwards
scrutinize the re-action of the interior organ or brain.

The eyes are very delicate, very movable organs, by means of which the
sensation of light or colour is experienced: these give to the brain a
distinct perception, in consequence of which, man forms an idea,
generated by the action of luminous or coloured bodies: as soon as the
eyelids are opened, the retina is affected in a peculiar manner; the
fluid, the fibres, the nerves, of which they are composed, are excited
by shocks which they communicate to the brain; to which they delineate
the images of the bodies from which they have received the impulse; by
this means, an idea is acquired of the colour, the size, the form, the
distance of these bodies: it is thus that may he explained the mechanism
of _sight_.

The mobility and the elasticity of which the skin is rendered
susceptible, by the fibres and nerves which form its texture, accounts
for the rapidity with which this envelope to the human body is affected
when applied to any other body; by their agency, the brain has notice of