Functional Morphology of the Eye
The eye is a complex organ responsible for vision. Understanding the functions of the visual system requires knowledge of the eye's structure. The major structures of the eye and their functions are as follows:
- Conjunctiva: The membrane that covers the front of the eye and lines the eyelid.
- Corneal Epithelium: An epithelial layer that covers and protects the cornea, resisting the free flow of fluids and bacteria.
- Cornea: The transparent front part of the eye that covers the iris, pupil, and anterior chamber. The cornea contributes most of the eye's focusing power and provides a protective function. It is fibrous and part of the outer layer of the eye.
- Aqueous Humor: A transparent fluid secreted from the ciliary epithelium that supports the iris. It maintains intraocular pressure and assists with the refractive index. The aqueous humor fills the anterior chamber of the eye.
- Iris: Controls the diameter and size of the pupil, and determines eye color. It connects to a sphincter papillae, which contracts the pupil, and dilator papillae, which pulls the iris out radially to enlarge the pupil. The iris and the choroid make up the vascular middle layer of the eye.
- Lens: A transparent, biconvex structure that refracts light to be focused on the retina. It changes shape, as a result of the ciliary muscles, to focus on objects at various distances (accommodation). The lens focuses light.
- Ciliary Muscle: A ring of multiunit smooth muscle that controls accommodation for viewing objects at various distances. It also regulates the flow of aqueous humor into the canal of Schlemm.
- Vitreous Humor: A clear gel that fills the space between the lens and the retina. The vitreous humor fills the posterior chamber of the eye.
- Retina: The retina contains cone cells and rod cells, which are light-sensitive, causing a cascade of chemical and electrical events that trigger nerve impulses. The retina is a neural layer and covers the entire posterior eye except for the blind spot, which is the optic disc (head of the optic nerve).
- Pigmented Epithelium: Responsible for light absorption, epithelial transport, spatial ion buffering, phagocytosis, secretion, and immune modulation. It is tightly associated with the underlying choroid.