The eye is the sensory organ of sight. It’s a hollow ball filled with fluid (vitreous humor) and consists of three layers:
• fibrous outer layer — sclera, bulbar conjunctiva, and cornea
• vascular middle layer — iris, ciliary body, and choroid
• inner layer — retina.
Sclera, bulbar conjunctiva, and cornea
The sclera is the white coating on the outside of the eyeball. Together with the vitreous humor on the inside, the sclera helps maintain the retina’s placement and the eyeball’s nearly spherical shape. The bulbar conjunctiva, a thin, transparent membrane that lines the eyelid, covers and protects the anterior portion of the white sclera. The cornea is a smooth, avascular, transparent tissue located in front of the iris that refracts (bends) light rays entering the eye. A film of tears coats the cornea, keeping it moist. The cornea merges with the sclera at the corneal limbus.
Iris and pupil
The iris is a circular, contractile diaphragm that contains smooth and radial muscles and is perforated in the center by the pupil. Varying amounts of pigment granules within the iris’s smooth muscle fibers give it color. Its posterior portion contains involuntary muscles that control pupil size to regulate the amount of light entering the eye.
The retina is the innermost layer of the eyeball. It receives visual stimuli and transmits images to the brain for processing. Vision of any kind depends on the retina and its structures. The retina contains the retinal vessels, the optic disk, the physiologic cup, rods and cones, the macula, and the fovea centralis. The retina has four sets of retinal vessels. Each of the four sets contains a transparent arteriole and vein that nourish the inner areas of the retina. As these vessels leave the optic disk, they become progressively thinner, intertwining as they extend to the periphery of the retina.