The eye is a complex sense organ used for vision. Vision is a special sense that is possible because of photoreceptor cells located in the back of the eye. Besides the photoreceptors, there are many structures inside and outside of the eye to aid in vision and eye protection. Accessory structures are not directly involved in vision but are essential to keeping the eye safe and functioning properly. The eyes are embedded in the ocular orbits of the skull and are cushioned by adipose tissue. The eyebrows and eyelashes are hairs that help prevent foreign materials from entering the eye. The eyelid is a sheet of skin and connective tissue that closes over the eye, protecting it from potential damage and keeping the eye from drying out. The inner eyelid secretes a solution of mucus, oil, and salts to keep the eye hydrated. The conjunctiva is a vascular mucous membrane that covers the inner surface of the eyelid, wraps around, and covers the anterior sclera, or whites, of the eye. This membrane functions to hydrate, protect, and reduce friction between the eye and the eyelid. The lacrimal gland, located at the medial corner, or outer edge, of the eye, is responsible for producing tears. Tears hydrate the eyes and help fight infection from the presence of bacteria-destroying enzymes.Extrinsic eye muscles are attached to the outer layer of the eye via tendons, and they allow directional change of the eyes. Four recti muscles allow upward, downward, and side-to-side movement. Two oblique muscles are attached at an angle and allow the eyes to rotate. These six muscles work together to allow the fine movements of the eyes in all directions. The structures of the eye include the interior of the eye and the eye wall. Three layers make up the wall of the eye. From outer to inner they are the fibrous, vascular, and neural tunics. The fibrous tunic includes the sclera and the cornea. The sclera is the white of the eye, and the cornea is the transparent anterior portion of the eye. The vascular tunic has three regions: the choroid, the ciliary body, and the iris. The choroid is the posterior pigmented region containing blood vessels. The lens is a biconvex transparent structure found posterior to the iris. The ciliary body forms the muscles that attach to the lens. When the muscles contract, the lens can change shape to help focus on objects that are near. The iris is the colored segment of the eye that can be seen through the cornea. The opening in the center of the iris is the pupil. The iris can dilate or constrict to change the diameter of the pupil, thereby controlling the amount of light entering the eye through the pupil. The neural tunic, or retina, of the eye contains a layer of pigmented cells, the photoreceptors, and the neurons, and at the back of the eye the neural tunic is continuous with the optic nerve.
The retina contains a layer of photoreceptors and two layers of sensory neurons. The photoreceptor layer is against the pigmented wall of the eye, short bipolar neurons make up the middle layer, and ganglion cells are neurons that make up the innermost layer. The axons of the ganglion cells converge at the optic disc and exit the eye through the optic nerve, which transmits visual information to the brain. Within the bipolar and ganglion cells are horizontal and amacrine cells that assist with visual processing.
The interior of the eye is divided into three chambers. The space between the cornea and the iris is the anterior chamber, and the space between the iris and lens is the posterior chamber. Behind the lens is the vitreous chamber. The inside of the eye contains two different types of fluid, the aqueous humor and vitreous humor. The aqueous humor is a watery solution secreted by the ciliary body that flows through the anterior and posterior chambers. The vitreous humor is a membrane-bound viscous fluid that fills the vitreous chamber.A number of other structures contribute to the eye's ability to see and perceive images. At the rear portion of the eye wall, the macula and fovea are parts of the retina. The macula is responsible for sharp vision. The fovea, also called the macula lutea, contains light-sensitive cells that provide central vision. In the anterior portion of the eye, the conjunctiva is a semitransparent film or membrane that lines the inside area of the eyelids. The conjunctiva protects the cornea.