Friday, February 11, 2011

Eye Dissection

Initial observation of the cow eye revealed that the eye was rather deflated and the optical nerve was hidden due to a large amount of fat and muscle tissue that covered the back half of the eye. The cornea, which is normally clear, had turned opaque due to the preserving fluids that were used and making the iris behind the cornea appear a bluish/purple hue. The first step was to remove the excess fat and muscle tissue around the eye. Removal of these tissues proved relatively easy given a sharp set of scissors. The fat tissue was greyish in color while the muscle tissue had a deep red color and was marbled with fat. Unlike the softer fat tissue, the muscle tissue was much harder and didn’t mold to pressure. After the fat and muscle tissue were removed, the sclera of the eye and optical nerve became visible. The sclera was whitish colored and rather tough. The optical nerve had the same tan/grey color of the fat and thus hid easily in the fat. It was about 5 mm in diameter and protruded from the back of the eye just over 5 mm as well. Cylindrical in shape, the optical nerve was rather tough and retained its shape against pressure.

The next step was to cut a slit into the sclera half way between the optical nerve and the cornea and continue to cut around the eye until the eye was in two separate halves. The sclera, although not particularly thick (2 mm at most) was tough to cut through. Upon separating the halves, the vitreous humor within the compartment of the eye became visible and through it, the lens. The vitreous humor was clear and gelatinous and occupied the space between the lens and retina. Although it originally was congealed into a distorted sphere within the eye, this shape was easily destroyed after it was removed. Along with the vitreous humor, there was a bit of clear liquid which would be the aqueous humor. I imagine that the vitreous and aqueous humors are necessary for keeping the eye inflated within the optic cavity and keeping the retina spread out along the back of the eye, but could also be useful in keeping light focused on the retina or providing some type of immunological protection for cells within the eye.

Looking at the back of the eye, the retina and the underlying choroid were visible. The retina was a thin layer of grey colored tissue that was easily destroyed. The retina, which is composed of all of the sensory neurons that perceive light entering the eye, was not attached to the choroid except in one spot. This singular spot, which is where the axons of all of the neurons exit the eye, is called the blind spot. Furthermore, this turns into the optic nerve which is the bundle of axons that cross in the optic chiasm and travel to the occipital lobe of the brain where the information is processed. The underlying choroid, which was a thin layer of cells attached to the inside of the sclera, was reflective and had a greenish/yellowish color to it. It is reflective so that any light entering the eye is reflected back onto the sensory neurons and is important for night vision. This also accounts for why cat’s eyes (and apparently cow’s eyes) reflect a light shone in them when in a dark room. Behind the choroid, the back of the sclera was black.

After the vitreous humor was removed, the front of the eye was inverted so that the lens popped out. It measured roughly 1.3 cm deep and 1.5 cm in diameter. The front side (touching the pupil and iris) was much flatter than the rounded back side, but it is known that the lens can change its shape in order to help focus incoming light on the retina. Due to the fluids used to preserve it, the lens was extremely hard. A razor was used to cut it in half, and I found that the lens had multiple layers. The outside was a glossy layer and underneath was a stringy layer that reminded me of string cheese when peeled off. Underneath this, the lens was much clearer and could be used to magnify words on a page. Once the lens was removed, the slit through the iris, the pupil, became visible, and through it the cornea. Detachable from the circular iris were the ciliary muscles, which hold the lens in place as well as change its shape through contraction. Upon removing these muscles (the iris and ciliary muscles) from the eye, all of the structures necessary for refracting light into the eye, focusing it and finally detecting and relaying the signal to the brain had finally been removed and observed.

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