Sheep Eye Dissection

Today in class, my lab partner and I dissected a sheep eye in order to fully understand the anatomy and physiology of the eye, although some features on the sheep eye are different from those of a human eye. First, we looked at the external surface of the eye, identifying fatty and muscle tissue that we had to remove.

Excess tissue is the white around the eye; we stopped cutting once we reached tough tissue (sclera).

After cutting away the excess tissue, we identified the cornea and sclera. We did not take a picture of this step, so here is a picture from online:

1. cornea

2. sclera

The cornea is the layer that forms the front of the eye and allows light to come in; the cornea is slightly cloudy because the eye is not in a living state.

We also identified the optic nerve, which is directly behind the eye:

1. optic nerve

The optic nerve transmits the signals to the brain so the brain can interpret the light coming in.

Then, we made an incision between the sclera and the optic nerve and cut the eyeball in half all the way around. When we separated the two hemispheres of the eye, we discovered the retina on the posterior hemisphere and the vitreous humor on the anterior hemisphere. The vitreous humor and the aqueous humor (fluid) maintains the shape of the eye. Underneath the retina was the thin, black choroid layer, which also contained a shiny, bluish structure called the tapetum lucidum. 

Lifting up the choroid layer from the sclera.

The choroid layer contains a network of blood vessels that bring nourishment and oxygen to itself and the other two layers.

Shiny, bluish material is the tapetum lucidum.

In order to fully expose the tapetum lucidum like in the picture above, we removed most of the retina, but kept the retina attached at its one point of attachment, which is the blind spot. The retina uses photoreceptors to send signals from the light to the brain through the optic nerve. The tapetum lucidum reflects light onto the retina, which helps animals with night vision; humans do not have this feature. 

Next, we removed the vitreous humor from the anterior hemisphere of the eye, revealing the lens, ciliary body, and suspensory ligaments. 

The small opening through all the black (iris) is the pupil, through which light enters the eye. The circular structure to the very right is the lens. Around the pupil, there is also the ciliary body, which is muscle fibers that contract and relax to increase or reduce the bending of the light rays. 

We then removed the lens and noted that it was slightly cloudy. When we removed the lens, the suspensory ligaments were still somewhat attached to the lens:

The suspensory ligaments are the black bits around the lens. These ligaments hold the lens in place and join with the smooth muscle containing the ciliary body.

We observed that the shape of the pupil was not completely circular like it is in humans, which is another difference between the sheep and human eye. After observing the iris and cornea, we removed the cornea from the front of the eye and noticed that it was tough, but definitely not as thick as the sclera. 

This lab was simple but offered many insights into the anatomy and physiology of the human eye!