f the
human eye.

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The vascular layer. 902. Where is the aqueous humor situated? What
part of the eye is called the anterior chamber? The posterior chamber?
With what are the chambers lined? 903. Where is the crystalline humor
situated? With what is it surrounded? Of what does the lens consist?
How are these layers best demonstrated? What is produced when the
lens, or its investing membrane, is changed in structure?

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2d. When the crystalline lens, or its investing membrane, is changed
in structure, so as to prevent the rays of light passing to the
retina, the affection is called a _cataract_.

[Illustration: Fig. 139. A section of the globe of the eye. 1, The
sclerotic coat. 2, The cornea (This connects with the sclerotic coat by a
bevelled edge.) 3, The choroid coat. 6, 6, The iris. 7, The pupil. 8, The
retina. 10, 11, 11, Chambers of the eye that contain the aqueous humor.
12, The crystalline lens. 13, The vitreous humor. 15, The optic nerve.
16, The central artery of the eye.]

904. The VITREOUS HUMOR forms the principal bulk of the globe of the
eye. It is an albuminous fluid, resembling the aqueous humor, but is
more dense, and differs from the aqueous in this important particular,
that it has not the power of re-producing itself. If by accident it is
discharged, the eye is irrecoverably lost; while the aqueous humor may
be let out, and will be again restored. It is enclosed in a delicate
membrane, called the _hy'a-loid_, which sends processes into the
interior of the globe of the eye, forming the cells in which the humor
is retained.

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904. Describe the vitreous humor. How does this humor differ from the
aqueous? What membrane encloses the vitreous humor?

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_Observation._ The structure of this organ can be seen by first
freezing the eye of a sheep or an ox; it then can be cut in various
directions, and each part separately examined.

905. The MUSCLES of the eye are six in number. They are attached, at
one extremity, to the bones of the orbit behind the eye; at the other
extremity, they are inserted by broad, thin tendons, near the junction
of the cornea with the sclerotic coat. The white, pearly appearance of
the eye is caused by these tendons.

[Illustration: Fig. 140. A view of the eye and its muscles. _a_, _b_,
_c_, _d_, _e_, Five of these muscles. _f_, The optic nerve. G, The
trochlea, o