Agriculture Reference
In-Depth Information
Figure 6.40
The
external structures
of the eye:
A. Frontal aspect of
the eye and
lacrimal apparatus;
B. Eyelids and the
eye.
nerve to exit the eyeball. The sclera continues anteriorly
with the transparent cornea, the fi rst refractive media of
the eye for the light beams.
The vascular tunic (the uvea) consists of the choroid, the
ciliary body, and the iris. The choroid lines intimately the
sclera. At the sclero-corneal junction, the choroid contin-
ues with the ciliary body (ciliary muscle and processes).
The ciliary muscle is responsible for the accommodation
of the lens, whereas the ciliary processes produce the
aqueous humor. The iris is an incomplete vertical wall
between the cornea and the lens and is perforated by the
pupil. Thus, the iris separates the space between the
cornea and the lens into two chambers: the anterior and
the posterior chambers, which communicate with each
other through the pupil and are fi lled with the aqueous
humor.
The nervous tunic is represented by the retina. It lines
the choroid intimately and continues anteriorly with the
nonpigmented epithelium of the ciliary body. The retina
contains sensory cells (receptors) that send the visual
excitation via the optic nerve to the brain to be processed
into visual sensations. The beginning of the optic nerve
is the optic disc, which lacks receptors. In 15-20% of
goats only, minute blood vessels covered by pigmented
epithelium of the optic disc can be seen, an important
characteristic while looking at the fundus of the eye with
the ophthalmoscope.
The lens is an elastic round and biconvex transparent
structure, held in place by a ligament from the ciliary body.
The space between the lens and the retina is called the
vitreous chamber, fi lled with the vitreous body. The latter
is a transparent refractive medium, with the aspect of a
fresh egg albumen. Thus, the refractive media of the eye
are the cornea, the aqueous humor, the lens, and the vitre-
ous body.
The
adnexa
(the accessory organs) of the eye consist of
fasciae, muscles, vessels and nerves, eyelids, and the lac-
rimal apparatus.
The fasciae are tough connective tissue structures
enveloping the eyeball, muscles, vessels, and nerves, and
sending septa between muscles. The muscles move the eye
in all directions. There are straight and oblique muscles of
the eyeball and the levator of the upper lid. The vessels are
mainly provided by the external ophthalmic artery, whereas
the nerves belong to the optic nerve (CN II), to the oculo-
motor, trochlear, and abducent motor nerves (CN III, IV,
