Eyeballs

The eyeball, or globe, is the sensory organ of vision. In humans it is roughly spherical with a diameter of about 24 millimeters and a mass around 7 to 8 grams. It sits in the orbits of the skull and is protected by the eyelids, eyelashes, and surrounding tissues.

The wall of the eyeball is organized into three layers. The outer fibrous tunic consists of the transparent cornea at the front and the opaque sclera at the back. The middle vascular or uveal tunic includes the iris, ciliary body, and choroid. The inner neural tunic is the retina, which contains the photoreceptors that detect light.

Internal compartments include the anterior segment, containing the anterior and posterior chambers filled with aqueous humor, and the posterior segment, filled with vitreous humor. The lens, suspended by zonules and anchored to the ciliary body, provides fine focusing of light. The pupil is the opening in the iris that regulates light entry.

Light travels through the cornea and aqueous humor, passes the pupil, is focused by the lens, and forms an image on the retina. Photoreceptors convert light into neural signals that travel via the optic nerve to the brain for processing. The fovea centralis in the center of the macula provides high-acuity, central vision.

The retina is nourished by the choroid and by retinal vessels. Visual information is transmitted through complex neural pathways to the brain, enabling perception and interpretation of shapes, colors, and motion. Common conditions affecting eyeballs include refractive errors (myopia, hyperopia, astigmatism), cataract, glaucoma, macular degeneration, and retinal detachment.