The eye is a complex and delicate organ that allows vertebrates and invertebrates to perceive visible light and convert it into nerve impulses, allowing them to see. The basic structures of the eye are the same in most species, although different species have adapted in different ways to their particular needs. For example, humans have developed binocular vision where both eyes face forward, while birds have one eye on each side of their heads.
Structure of the Eye
The eye is divided into three main layers. The main, outer layer is the fibrous tunic and is the only complete layer in that it encircles the entire structure of the eye. The middle layer is the vascular tunic, a network of blood vessels that provide oxygen and nutrients to the eye's tissues. The innermost layer is the nervous tunic, more commonly known as the retina, a layer of photoreceptive cells within the cavity of the eye.
The Fibrous Tunic
The fibrous tunic is the tough, outer layer of the eye. As the only complete layer, it provides protection to the inner mechanisms of the eye, helps it hold its shape and provides a surface for the attachment of the muscles responsible for moving the eye. The fibrous tunic is divided into two sections. The largest portion, the sclera, wraps around the back and sides of the eye, while the smaller cornea is located at the front.
The sclera is a firm, opaque membrane that forms approximately one-sixth of the total area of the fibrous tunic. The sclera is formed from collagen and fibrous connective tissue and is where the tendons and muscles of the eyeball attach. The very back of the sclera has a weaker plate where the fibers of the optic nerve pass through it. A small portion of the sclera can be seen as the "white" of the eye.
The cornea forms the frontal portion of the fibrous tunic and is made up of five extremely thin layers of cells arranged in such a way that the membrane is transparent. This allows light to enter the eye and reach the retina. The cornea bulges outward slightly and usually has a spherical curve. The eye condition astigmatism is caused by the cornea curving too far. The cornea is covered by a thin, transparent layer known as the conjunctiva.
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