[Illustration:
FIG. 560.—INNER SURFACE OF ANTERIOR PART OF EYEBALL OF HORSE
(EQUATORIAL SECTION).
_1_, Sclera; _2_, chorioidea; _3_, retina (drawn away from
chorioidea); _4_, ciliary processes; _5_, crystalline lens, through
which the pupil (_6_) is seen. (After Ellenberger, in Leisering’s
Atlas.)
]
[Illustration:
FIG. 561.—FUNDUS OCULI, SEEN ON EQUATORIAL SECTION OF EYEBALL OF
HORSE.
_1_, Sclera; _2_, chorioidea; _3_, retina (loosened); _4_, tapetum;
_5_, optic papilla; _6_, optic nerve. (After Ellenberger, in
Leisering’s Atlas.)
]
The =retina= or nervous tunic of the eyeball is a delicate membrane
which extends from the entrance of the optic nerve to the margin of the
pupil. It consists of three parts. The large posterior part, which alone
contains the nervous elements, including the special neuro-epithelium
and the optic nerve-fibers, is termed the =pars optica retinæ=. It
extends forward to the ciliary body, where it terminates at an irregular
line called the =ora serrata=. Here the retina rapidly loses its nervous
elements, becomes much thinner, and is continued over the ciliary body
and the posterior surface of the iris by two layers of epithelial cells.
In the =pars ciliaris retinæ= the inner stratum is non-pigmented, while
the outer layer is a direct continuation of the stratum pigmenti of the
pars optica. In the =pars iridica retinæ= both are pigmented. In the
dead subject the pars optica is an opaque, gray, soft membrane which can
be stripped of the chorioid, leaving most of its outer pigmented layer
on the latter. During life it is transparent, except as to its pigmented
epithelium, and the reddish appearance of the fundus as viewed by the
ophthalmoscope is caused by the blood in the network of the
choriocapillaris. The entrance of the optic nerve forms a sharply
defined, oval, light area, the =optic papilla= (Papilla nervi optici),
situated about 15 mm. below the horizontal meridian and 3 to 4 mm.
external to the vertical meridian. The central part of the papilla is
slightly depressed (Excavatio papillæ n. optici).
The transverse diameter of the papilla is about 6 to 7 mm., and the
vertical about 4 to 5 mm. It is commonly situated a little below the
margin of the tapetum, but the latter may extend down somewhat on
either side of the papilla. The lower margin is often indented a
little. In inspection of the fundus with the ophthalmoscope numerous
fine branches of the arteria centralis retinæ are seen radiating from
the periphery of the papilla.
The optic nerve-fibers converge from all parts of the pars optica to the
papilla, where they collect into bundles which traverse the lamina
cribrosa of the chorioidea and sclera, and constitute the optic nerve.
The =area centralis retinæ= is a round spot, 2 to 3 mm. in diameter,
situated above and external to the optic papilla; it corresponds to the
macula lutea of man, which is histologically more highly differentiated
than the rest of the retina and is the area of most acute vision.
The =structure= of the retina is very complex. It consists of nervous
elements which are supported in a peculiar sustentacular tissue, and are
covered externally by a layer of pigmented epithelium (Stratum pigmenti
retinæ). The nervous elements comprise a highly specialized
neuro-epithelium, the rods and cones; ganglion cells, the axones of
which form the optic nerve; and intermediate neurones. Ten layers may be
recognized in sections microscopically.[220]
The =arteries= of the retina are derived from the arteria centralis
retinæ and anastomotic branches from the short ciliary arteries. The
arteria centralis enters the optic nerve a short distance behind the
eyeball and runs in the axis of the nerve. The artery divides 2 to 3 mm.
before reaching the papilla, and gives off thirty to forty branches
which radiate in the posterior part of the retina and divide
dichotomously into end-arteries. The anterior part of the retina is
non-vascular. The veins accompany the arteries except in the capillary
plexuses; their walls consist merely of a layer of endothelial cells,
around which are a lymph-channel and sheath.