Eye Dissection Analysis: How do we see things?

The un-dissected sheep eye. Baa.

Did you know that the image formed on our retina of everything we see is actually upside down? You may be thinking, "How do people see normally without constantly doing a handstand, then?" I shall explain this and the wonderful process that is vision now, through the aid of a dissected sheep's eye.

Our sheep had a beautiful blue iris.

The tapetum lucidum reflects the flash off my camera.

When light enters our eye, it passes first through the cornea. This is the tough exterior on the eye, which mainly serves as protection. Next, the image passes through the aqueous humor, which is a clear liquid in between the cornea and the lens. You can see the liquid on the mat of the dissection board, as it leaked out after the cornea was punctured. The image then passes through the pupil, which contrary to what you may think, is not a tangible black dot in the center of your eye. The pupil is actually a hole, an opening in the iris that allows light to pass through. The iris is the colored part of the eye, and it comes in many colors such as brown, blue, green, hazel, and purple if you're Elizabeth Taylor. The image passes through the pupil opening onto the lens, a very interesting structure that can actually change shape! The lens changes shape in order to focus light on the retina. This is what you see when someone's "pupils dilate." Their pupils seem to grow in size, which happens to allow more light in while in a darker environment. In sheep eyes, there is a secondary feature to allow even more light to be reflected into the retina at night called the tapetum lucidum. This is the reflective and iridescent part on the inside of the eye. After passing the lens, the light passes through another clear liquid called the vitreous humor, which is the jelly like substance sliding out of the eye in the attached pictures. The image then hits the retina, which contains the photoreceptors for vision, and is displayed upside down. Now, the brain does some pretty remarkable work to interpret the image. Where the retina and the optic nerve meet is a small divet called the "blind spot." The electrical signals are sent via the optic nerve to the occipital lobe, located in the back of the head. In the occipital lobe, vision is interpreted and flipped through complex tasks in order to match our perception with reality. Some studies say that at birth, babies still see the world upside down until the brain adjusts and corrects itself!