Dissection of Eye

By: B. Abbrasov

Introduction
An eye is a light-sensitive organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark in complex organs, such as those of humans and other mammals that can distinguish minute variations of shape, color, brightness and distance. The actual process of seeing the electromagnetic vibrations of light into patterns of nerve impluses that are transmitted to the brain. All mammalian eyes are very similar to structure; therefore dissecting a sheep eye is an excellent activity for learning human eye structure. Preserved eyes are somewhat shrunken and deformed from the preservative, but the important anatomical features are retained.

Procedure
1) Clean the fat off the eye.
2) Open the eye by making a frontal section through middle of the eyeball, separating it into anterior and posterior halves.
3) Remove the vitreous body slowly, being careful not to tear away the thin, grayish retrina attached to the inner, posterior surface.
4) Take the lens of the eye and push it down with fingers.
5) Try to take out the retina.
6) Identify the parts of the eye and draw diagrams.

Definitions
1) Sclera - the tough ineclastic opaque membrane covering the posterior five sixths of the eye bulb. It maintains the size and form of the bulb and attaches to muscles that move the bulb. Posteriorly it is pierced by the optic nerve and, with the transparent cornea, makes up the outermost of three tunics covering the eye bulb.

2) Cornea - the convex, transparent anterior part of the eye bulb. It allows light to pass through it to the lens. The cornea is a fibrous structure with five layers: the anterior corneal epithelium, continuous with that of the conjunctiva; the anterior limiting layer (Bowman's membrane); the substania propria; the posterior limiting layer (Descemet's membrane); and the endothelium of the anterior chamber (keratoderma). It is dense, uniform in thickness, and nonvascular, and it projects like a dome beyond the sclera, which forms the other five sixths of the eye's outermost tunic.

3) Choroid - a thin, highly vascular layer of the eye between the retina and sclera.

4) Ciliary Body - the thickened part of the vascular tunic of the eye that joins the iris with the anterior portion of the choroids. It is composed of the ciliary crown, ciliary processes and folds, ciliary orbiculus, ciliary muscle, and a basal lamina.

5) Iris - an annular contractile disk suspended in aqueous humor between the cornea and the crystalline lens of the eye enclosing a circular pupil. Smooth muscle fibers of the iris contract and relax to allow more or less light to enter the eye through the pupil.

6) Retina - a 10 layered delicate nervous tissue membrane of the eye, continuous with the optic nerve that receives images of external objects and transmits visual impulses through the optic nerve to the brain. The retina is soft and semitransparent and contains rhodopsin. It consists of the outer pigmented layer and the nine-layered retina proper.

7) Extraocular Muscles - the six sets of muscles that control movements of the eyeball. They are the superior rectus and inferior rectus, which move the eye up and down; the medial rectus and the lateral rectus, which move the eye to both sides, and the superior oblique and inferior oblique, which move the eye upward and outward, and downward and outward.

8) Lens - a curved transparent piece that refracts light in a specific way; the crystalline lens of the eye.

9) Vitreous Chamber - the cavity in the eye posterior to the lens that contains the vitreous body and vitreous membrane and is transected by the vestigial remnants of the hyaloid canal.

10) Anterior Chamber - the part of the anterior cavity of the eye in front of the iris.

11) Posterior Chamber - located between the iris, zonule fibers, and the lens, filled with aqueous humor.

12) Optic Nerve - one of a pair of cranial nerves that transmits visual impulses. It consists mainly of course myelinated fibers that arise in the retinal ganglionic layer, traverse the thalamus, and connect with the visual cortex.

13) Optic Disk - the small blind spot on the surface of the retina, located about 3 mm to the nasal side of the macula. It is the point where the fibers of the retina leave the eye and become part of the optic nerve. It is the only part of the retina that is insensitive to light.

14) Fovea - an area at the center of the retina where cone cells are concentrated and there are no rod cells.

15) Conjunctiva - the mucous membrane lining the inner surfaces of the eyelids and anterior part of the sclera.

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