two roots from the central portion of
the base of the brain. The two nerves approach each other, as they
proceed forward, and some of the fibres of each cross to the nerve of
the opposite side. They then diverge, and enter the globe of the eyes
at their back part, where they expand, and form a soft, whitish
membrane.

893. The GLOBE, or ball of the eye, is an optical instrument of the
most perfect construction. The sides of the globes are composed of
_Coats_, or membranes. The interior of the globe is filled with
refracting _Humors_, or _me'di-ums_.

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890. Which sense contributes most to the enjoyment of man? What do we
perceive by this sense? What is said of the mechanism of the eye?
891-916. _Give the anatomy of the organs of vision._ 891. Of what does
the apparatus of vision consist? 892. Describe the optic nerve. 893.
Describe the globe of the eye.

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894. The COATS are three in number: 1st. The _Scle-rot'ic_ and
_Corn'e-a_. 2d. The _Cho'roid_, _Iris_, and _Cil'ia-ry processes_. 3d.
The _Ret'i-na_.

895 The HUMORS are also three in number: 1st. The _A'que-ous_, or
watery. 2d. The _Crys'tal-line_, (lens.) 3d. The _Vit're-ous_, or
glassy.

[Illustration: Fig. 137. The second pair of nerves. 1, 1, Globe of the
eye: the one on the left is perfect, but that on the right has the
sclerotic and choroid coats removed, to show the retina. 2, The crossing
of the optic nerve. 5, The pons varolii. 6, The medulla oblongata. 7, 8,
9, 10, 11, 12, 13, The origin of several pairs of cranial nerves.]

896. The SCLEROTIC COAT is a dense, fibrous membrane and invests about
four fifths of the globe of the eye. It gives form to this organ, and
serves for the attachment of the muscles that move the eye in various
directions. This coat, from the brilliancy of its whiteness, is known
by the name of "the white of the eye." Anteriorly, the sclerotic coat
presents a bevelled edge, which receives the cornea in the same way
that a watch-glass is received by the groove in its case.

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894. Name the coats of the eye. 895. Name the humors of the eye.
Explain fig. 137. 896. Describe the sclerotic coat.

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897. The CORNEA is the transparent projecting layer, that forms the
anterior fifth of the globe of the eye. In form, it is circular,
convexo-concave, and resembles a watch-glass. It is received by its
edge, which is sharp and th