Eyeball Structure and Function

Study Notes

The human eyeball is roughly spherical, about 24 mm in diameter.
It's slightly flattened from top to bottom.
The eyeball is divided into an anterior and posterior segment.
The anterior segment is smaller and comprises about one-sixth of the eyeball.
The posterior segment is larger, encompassing five-sixths of the eyeball.
The posterior wall of the eyeball is lined by the retina, which is light-sensitive.

The center of the anterior curvature is called the anterior pole, and the center of the posterior curvature is called the posterior pole.
The line connecting these two poles is the optic axis.
The line connecting a point in the cornea, slightly inward from the anterior pole, and the fovea centralis (situated lateral to the posterior pole) is the visual axis.
Light rays passing through the visual axis of the eyeball are not refracted (bent).
The optic nerve exits the eye slightly medial to the posterior pole.

The eyeball, excluding the anterior one-sixth, resides in the bony orbital cavity or eye socket.
A layer of fatty areolar connective tissue, acts as a cushion between the bone and eyeball.
This cushioning tissue protects the eyeball from external force.
Six extra-ocular muscles attach the eyeball to the orbital cavity.

Eyelids protect the eyeball from foreign particles and block light during sleep.
Eyelid closure is both voluntary and reflexive.
The average person blinks about 20 times per minute.
Blinking keeps the eyes healthy by delivering oxygen, maintaining moisture, and removing debris.
The eyelid margins have sensitive hairs called cilia or eyelashes.
Each cilium originates from a follicle surrounded by a sensory nerve plexus.
When dust particles touch cilia, the sensory nerves are activated, leading to rapid blinking which prevents the particles reaching the cornea.

The eyelid muscle retractors open the eye.
Retractors of the upper eyelid include:
- Levator palpebrae superioris (cranial nerve III)
- Müller's muscle (sympathetic supply)
- Frontalis (cranial nerve VII)
The lower eyelid retractors include:
- Capsulopalpebral muscle (cranial nerve V)
- Inferior tarsal/palpebral muscle (cranial nerve V)

The orbicularis oculi is a skeletal facial muscle found in both upper and lower eyelids.
It is responsible for closing the palpebral fissure (eyelid opening) and is supplied by cranial nerve VII (Facial nerve).

Meibomian glands and sebaceous glands are present in the eyelids.
These glands open into the cilia follicles.
Infection of these glands leads to a painful, pus-filled lesion called a sty, which is a small, acute abscess.

The elliptical opening between the two eyelids is called the palpebral fissure.
The average width of the palpebral fissure is 31 mm in males and 29 mm in females.