ssion of the rays
of light which leave an object at which we look, and it is upon it
that a small but very clear image of that object is formed. The
impression thus produced by the reflected light is transmitted by the
optic nerve to the brain, which receives the sensation. This
constitutes vision.

924. The optic nerve has but one function, that of sight. Sensibility
is conferred on this organ by a large branch from the fifth pair of
nerves, which ramifies upon the different parts of the eye and its
appendages. These parts, however, receive some nervous filaments from
the seventh pair.

_Observations._ 1st. The large number of sensitive nervous filaments
renders the visual organ very impressible to bodies that cause
irritation, as dust, or intense light. This compels us to use due care
to shield the eye from the influence of agents that would impair or
destroy vision.

2d. Although particles of dust, when in contact with the delicate
parts of the eye, induce severe pain, yet these parts may be cut in
surgical operations, and the patient's sufferings are not as great as
when an incision is made in the skin to remove a small tumor.

925. Different degrees of density, as already mentioned, modify the
refractory power of any transparent medium. It is found, on
examination, that the cornea, the vitreous, the crystalline, and the
aqueous humors, have each, severally, various degrees of density: and
that the crystalline lens, at its circumference, is less dense than at
its centre. These circumstances modify the direction of the refraction
of the rays of light, in their passage from the cornea to the retina.

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922. What is the use of the cornea, aqueous, crystalline, and
vitreous humors? 923. What is the office of the retina? 924. What is
the function of the optic nerve? How is sensibility conferred on
this organ? Give the 1st observation in this connection. The 2d
observation.

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926. The refracting powers of the plane, convex, concave, plano-convex,
plano-concave, and concavo-convex lenses,[22] are different. The
cornea and aqueous humors are convexo-concave, the vitreous humor is
concavo-convex, while the crystalline humor is a convexo-convex medium.
(Fig. 139.)

[22] The refracting character of differently-formed lenses is
illustrated in the works on Natural Philosophy, to which the
pupil is referred.

[Illustration: Fig. 142. The forms of