s: the surface presented to view in this section being the uvea. 4,
The ciliary processes. 5, The scalloped anterior border of the retina.]

901. The RETINA is composed of three layers: The external; middle, or
nervous; and internal, or vascular. The external membrane is extremely
thin, and is seen as a flocculent film, when the eye is suspended in
water. The nervous membrane is the expansion of the optic nerve, and
forms a thin, semi-transparent, bluish-white layer. The vascular
membrane consists of the ramifications of a minute artery and its
accompanying vein. This vascular layer forms distinct sheaths for the
nervous papillae, which constitute the inner surface of the retina.

-=-=-=-=-=-=-=-=-=-=-=-=

900. How are the ciliary processes formed? What does fig. 138 exhibit?
901. Of how many layers is the retina composed? Describe the external
layer. The nervous layer.

-=-=-=-=-=-=-=-=-=-=-=-=

902. The AQUEOUS HUMOR is situated in the anterior and posterior
chambers of the eye. It is an albuminous fluid, having an alkaline
reaction. Its specific gravity is a very little greater than distilled
water. The anterior chamber is the space intervening between the
cornea, in front, and the iris and pupil, behind. The posterior
chamber is the narrow space, less than half a line in depth, bounded
by the posterior surface of the iris and pupil, in front, and by the
ciliary processes and crystalline lens, behind. The two chambers are
lined by a thin layer, the secreting membrane of the aqueous humor.

903. The CRYSTALLINE HUMOR, or lens, is situated immediately behind
the pupil, and is surrounded by the ciliary processes. This humor is
more convex on the posterior than on the anterior surface, and, in
different portions of the surface of each, the convexity varies from
their oval character. It is imbedded in the anterior part of the
vitreous humor, from which it is separated by a thin membrane, and is
invested by a transparent elastic membrane, called the capsule of the
lens. The lens consists of concentric layers, disposed like the coats
of an onion. The external layer is soft, and each successive one
increases in firmness until the central layer forms a hardened
nucleus. These layers are best demonstrated by boiling, or by
immersion in alcohol, when they separate easily from each other.

_Observations._ 1st. The lens in the eye of a fish is round, like a
globe, and has the same appearance, when boiled, as the lens o