04.26.2016 Sheep Eye Dissection

Today we dissected a sheep's eye to help us learn about the anatomy and physiology of an eye.

a sheep's eye

First, we studied the external structures of the eye. These included the eyelid, the cornea (clear part of the eye to let light in), the sclera (white of the eye), extrinsic eye muscles that move the eye around, fatty tissue that cushions the eye, and the optic nerve that sends visual information to the brain.

optic nerve

extrinsic eye muscle

Then we cut the eye open to study the internal structures. As we cut through the sclera, we found that it was very thick and tough and hard to cut through. Also, a liquid oozed out of the eye as we cut. This liquid, called the vitreous humor, has a jelly-like consistency and is also clear to let light through. On the inside of the posterior hemisphere of the eye, we found the retina, which is a very thin layer of sensory cells that receives light and converts it to nerve impulses that they send to the brain through the optic nerve. Underneath the retina is a dark-colored layer called the choroid layer, which brings nutrients and blood to the retina. It has a dark color so that it does not reflect light around the eye. In some nocturnal animals (i.e. not humans), the part of the choroid layer is bluish and reflective. This is called the tapetum lucidum, which reflects light around the eye and allows the animals to see better in the dark.

the choroid layer and the tapetum lucidum

the thickness of the sclera

In the front of the eye, we found the lens attached to the ciliary body by suspensory ligaments. The lens and the ciliary body, which is made of smooth muscle, work together to allow the eye to focus on an object. The ciliary body can contract to flatten the lens and reduce the bending of light rays entering the eye, or it can relax to allow the lens to thicken and increase the bending of light rays entering the eye.

We also removed the cornea from the front of the eye to expose the iris and the pupil. The iris is the colored part of the eye, and the pupil is the hole in the middle. The pupil is what regulates the amount of light entering the eye by constricting or dilating.

back of the lens showing where the suspensory ligaments once attached (black parts)

the cornea, iris, and pupil