The Eye: Dissection of the Steer or Pig Eye

1. Begin by examining the whole eye, noting how the conjunctiva is attached to the sclera, and how the optic nerve, covered by its fibrous sheath (dura), enters the back of the globe.  Note the position of the extrinsic muscles.  Note how much of the eye is covered by conjunctiva. Remove the conjunctiva, fat and muscles from the globe with scissors.
2. Hold the globe in your hand between index finger and thumb.  The border between the cornea (transparent) and sclera (white) is called the limbus.  When you squeeze gently, there is a "whitening effect" of the cornea due to disruption of the ordered collagen arrays in the corneal stroma causing temporary loss of corneal transparency. 
3. With the posterior side of the globe resting on a piece of foil, make a small incision with razor blade about 4mm below the limbus.  Using scissors, continue this cut around the globe, parallel to the limbus, cutting through the conjunctiva, sclera, choroid and retina.  Note the loss of aqueous humor (anterior fluid) when the globe is punctured.
4. Lift the anterior portion gently on one side, and note the vitreous attachment to the lens capsule (the lens capsule will probably not be visible at this point).
5. Using scissors, cut across the vitreous posterior to the lens to release the anterior segment of the globe from the posterior segment.
   
   Anterior Segment of the Eye
   
6. Place the anterior portion of the eye (with the cornea down) on the foil.  Note the thickness and toughness of the cornea as well as its clarity and curvature.
7. Looking into the anterior globe, note the relatively large   area of the lens covered by the iris.  The anterior part of the iris consists of connective tissue containing melanocytes (pigmented cells) whose density determines eye color.  The posterior surface of the iris consists of 2 layers of pigmented epithelial cells.  Try to visualize how the sphincter and dilator muscles of the iris operate.   Make several radial cuts in the iris to expose the lens and allow examination of the zonules.  Examine the underlying ciliary body and processes (finger-like projections), just posterior to the iris.  Visualize how changes in the circular ciliary muscle might affect the tension on the zonules and result in changes in lens curvature (accommodation).
8. Remove the lens by cutting the zonular fibers (you will not be able to see these; simply cut around the periphery of the lens).  Lift the lens with partially opened scissors.  It will be attached to the vitreous at the posterior side (the ligament of Wieger).  Be careful not to puncture it, as this will change its transparency.
9. Note the elasticity and relative toughness of the lens.  Hold the lens over any typed letters to note its magnifying power.  Examine the outer cortical and inner nuclear layers.
10. Lift away the anterior vitreous from the ciliary body using forceps, and note the ciliary attachments remaining on the vitreous.  Note the differences in morphology of the two parts of the ciliary body, the pars plicata (pleated or plicated nearer the iris) and the pars plana (smooth, adjacent to the ora serrata).  The ora serrata is the scalloped edge of the pars plana at its junction with the neural retina.
11. Gently push away the ciliary body and iris (black in appearance) from the cornea with its scleral ring.  Note the ciliary processes, which appear as ridges surrounding the iris posteriorly. 
    
    Posterior Segment of the Eye
    
12. Looking down into the globe, examine the retina.  Note the iridescence due to the tapetum lucidum and the black color due to the tapetum nigrum.  (Both are located within the choroid and are unique to animals requiring night vision.)  Observe the optic disc (papilla or nerve head), which is the point of attachment/exit of the optic nerve and the morphology of the retinal vessels.
13. Gently slide the vitreous from the posterior "cup" and note its consistency.
14. After the removal of the vitreous, cut the "cup" remaining in half.  Do not cut through the optic disc.  In cross section note: sclera, choroid, retinal pigment epithelium, neural retina (innermost; this fragile tissue usually becomes folded when the vitreous is removed).
15. Using forceps, clear the retina from the inner surface on the portion of the "cup" which has the optic disc intact.  Note the attachment of the retina at the optic nerve insertion.

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