Sheep Eyeball Dissection Procedure

Sheep eyeball dissection procedure 1- Dry the eye with paper toweling. Examine the front of the eye and locate the cornea, sclera and fatty tissue. Examine the back of the eye and find extrinsic muscle bundles, fatty tissue and the optic nerve. If the optic nerve is not visible use the probe to move the fatty tissue around until the nerve is exposed. 2- Place your eye specimen in the dissection pan. Turn the specimen so the cornea is on the left and the optic nerve is on your right. Select a place to make an incision of the sclera midway between the cornea and optic nerve. Use the scalpel to make a small cut through the sclera. Fluid should ooze out of the eyeball when you have cut deeply enough. You will be reminded of how tough the sclera is when you make this cut. 3- Insert the point of the scissors into the slit made by the scalpel and cut the sclera with a shallow snipping motion. Cut the sclera all the way around the eyeball (cut into anterior and posterior hemispheres). 4- Observe the gelatinous vitreous humor that fills the central cavity of the eye. 5- Use forceps (probe or tip of scissor) to peel the retina away from the underlying choroid coat. The retina should remain attached at the blind spot. The choroid coat is dark and relatively thin. Use forceps (probe or tip of scissor) to gently separate the choroid from the outer sclera. Verify that the eye has three distinct layers, the retina, choroid and sclera. 6- Use forceps (probe or tip of scissor) to remove the vitreous humor from the anterior hemisphere of the eye. Try not to disturb the lens that is just below the vitreous humor. 7- Removal of the vitreous humor reveals the lens, ciliary body and suspensory ligaments. Remove the lens by pulling it free from its attachments. Note the shape of the lens, its stiffness and opaqueness. 8- When the lens is removed, an opening, allowing light to enter the eye is seen. This opening, the pupil is located in the center of the iris. 9- Remove the cornea from the anterior eye hemisphere. Use a scalpel to puncture a small slit at the boundary between the cornea and sclera. Then insert the scissors into the slip and cut all the way around the cornea to remove it. Notice the thickness of the cornea. How does it compare to the thickness of the sclera? Carefully observe the anterior side of the iris and pupil. Which structure of the eye would be just behind the pupil opening? 10- Observe the eye model. Examine the external muscles. Notes:  Sheep have four extrinsic muscles (humans have six)  The fatty tissue cushions the eye  The cloudy nature of the cornea is caused by the non-living tissue. It is transparent in the living state  The vitreous humor along with the aqueous humor (found behind the cornea) helps to maintain the shape of the eye.  Notice that the retina is only firmly attached to the choroid at one place. This region is the optic disc or blind spot. Here the nerve fibers leave the retina and form the optic nerve which is directly behind the blind spot.  The choroid contains an extensive network of blood vessels that bring nourishment and oxygen to itself and the other two layers. The dark color, caused by pigments, absorbs light so that it is not reflected around inside of the eye.  The tapetum lucidum, which is not found in the human eye, functions to reflect light onto the retina. It especially helps animals with night vision since it can reflect light even at very low intensities. It is shiny, glittering with a bluish color.  In the normal condition the lens is transparent except, when as a condition of aging, the lens turns cloudy. The cloudy condition, called cataract, prevents or reduces the amount of light reaching the retina. The normal lens is convex shaped and somewhat elastic. It is held in place by the suspensory ligaments that in turn join with the smooth muscle containing ciliary body (ciliary muscle).  Two muscle layers of the iris regulate the size of the pupil. One layer increases the pupil size with decreasing light intensity and the other layer reduces pupil size with increasing light intensity (sphincter and dilator pupillary muscles).  Note the oblong shape of the sheep pupil; in humans the pupil is circular. Suspensory ligaments