Sight is a nervous sensation due to the translation by the brain of the
effects caused by rays of light being reflected from some object in
front of the eye on to the innermost layer of the eye, the retina.
When an object is looked at, rays of light which reach the object from
some source of light (such as the sun, a lamp, etc.) fall on the
transparent outer part of the eye, the cornea. On account of its curved
surface these rays of light are more or less bent inward so as to fall
more or less perpendicularly on the forward anterior convex surface of
the lens. If the light is weak or dim, the iris, which lies in front of
the lens, will automatically contract down so as to make the opening by
which the rays can enter the posterior chamber of the eye (the part
behind the lens) as large as possible.
If the light is very bright the muscle fibers in the iris will relax so
that the iris itself gets larger, and its central opening smaller, so
that too much light may not enter. Passing through the lens the rays are
focused by the lens so that they are brought together to a point exactly
on the surface of the retina.
Here their presence has a certain effect on the rod and cone layer of
the retina, the result of which is conducted along the optic nerve to
the brain, where it is transformed into what we know as sight.
[Illustration: =HOW WE ARE ABLE TO TASTE, SMELL AND FEEL=
=HOW THE NERVES RUN INTO THE SPINAL CORD ON THE WAY TO THE BRAIN=
The lower drawing shows how the spinal cord rests in the backbone, and
how the nerves pass in and out, those of sensation passing into the
spinal cord, as shown in the magnified section above.]
[Illustration: =THE AREAS OF THE TONGUE IN WHICH THE CELLS OF TASTE ARE
DISTRIBUTED=
The tongue is covered with various types of taste-bulbs, most of the
distinct types that appreciate the sweet, the acid, and the bitter being
found in the areas marked on the diagram.]
[Illustration: =THE OUTER SIDE OF THE NOSE, SHOWING THE NERVES OF SMELL
AND FEELING=]
[Illustration: =THE INNER PART OF THE NOSE, SHOWING THE FIBRES FROM THE
OLFACTORY BULB=]