HealthKnow

General description of the eyeball

Morphology

Human eyeball (bulbus oculi) is approximately globe shaped with a diameter of about 24mm. It is slightly flattened from above downwards. Eyeball is made up of two segments, an anterior part and a posterior part. Anterior part is small and forms 1/6 of the eyeball. The posterior part is larger and forms 5/6 of the eyeball. The radius of this is about 8mm. The posterior wall of this part is lined by the light sensitive structure called retina.

The center of anterior curvature of the eyeball is called the anterior pole, and the center of posterior curvature is called the posterior pole. The line joining the two poles is called optic axis. The line joining a point in cornea little medial to anterior pole and the fovea centralis situated lateral to posterior pole is known as visual axis. The light rays pass through the visual axis of eyeball. The optic nerve leaves the eye, little medial to the posterior pole.

Orbital cavity

Except the anterior 1/6, the eyeball is situated in the bony cavity known as orbital cavity or eye socket. A thick layer of areolar tissue is interposed between the bone and eye. This serves as a cushion to protect the eyeball from external force. The eyeballs are attached to orbital cavity by the ocular muscles.

Eyelids

Eyelids protect the eyeball from foreign particles coming in contact with its surface and cut off the light during sleep. The eyelids can be closed voluntarily as well as reflexly.

The margins of eyelids have sensitive hairs called the cilia. Each cilium has a follicle, which is surrounded by a sensory nerve plexus. When the dust particle comes in contact with cilia, these sensory nerves are activated resulting in rapid blinking of eyelids. This prevents the dust particles from reaching the eyeball. There are about 100 to 150 cilia in the upper eyelid and about 50 to 75 in the lower lid. Meibomian glands and some sebaceous glands are also situated in the eyelids. These glands open into the follicles of cilia. The infection of these glands leads to the development of common eye sty.

Conjunctiva

It is a thin mucus membrane, which covers the exposed part of the eye. After covering the anterior surface, the conjunctiva is reflected into the inner surfaces of the eyelids. The part of conjunctiva covering the eyeball is called the bulbar portion. The part covering the eyelid is called the palpebral portion. When the eyelids are closed or opened, the opposed portions of conjunctiva slid over each other. The surface of conjunctiva is lubricated by thin film of tears secreted by lacrimal gland.