Eyeball Anatomy and Functions

Overview

This lecture covers the basic anatomy and key physiological functions of the eyeball, focusing on its three main layers and associated structures.

The purpose of the eye is to form clear images on the retina.
The eyeball consists of three main layers: outer coat, middle coat, and inner coat.

The outer coat provides protection and is made up of the sclera and cornea.
The sclera is the white, opaque part of the eye and comprises about 93% of the surface.
The cornea is transparent, covers about 7% of the surface, and allows light to enter the eye.
The smooth sclera serves as an attachment point for extraocular muscles.
The sclera has holes for blood vessels and is made of epithelial tissue.
The cornea has five layers, with the outermost being epithelial and unable to regenerate except via stem cells at the limbus (cornea-scleral junction).

The middle coat supplies blood and nutrients and has three parts: choroid, ciliary body, and iris.
The choroid is a thin, highly vascular layer lining the inside of the sclera.
The ciliary body thickens anteriorly, attaches to the lens via suspensory ligaments, and helps change lens shape for focus.
The ciliary body produces aqueous humor.
The iris (diaphragm) controls pupil size, thus regulating light entry.

The inner coat is the retina, with two main layers: the pigment layer (outer) and neural layer (inner).
The pigment layer contains melanin and is attached to the choroid, ciliary body, and iris.
The neural layer contains the eye's photosensitive cells (rods and cones) and is responsible for converting light into electrical signals.
The neural layer is only present in the posterior portion of the eye.

The vitreous humor is a gel-like substance filling the eye’s main body, maintaining its shape.
The aqueous humor is a watery fluid produced in the ciliary body, circulating between the posterior and anterior chambers and drained via a lymphatic vessel.
Poor aqueous humor drainage increases intraocular pressure, potentially causing glaucoma.

Sclera — Opaque, white outer layer of the eye.
Cornea — Transparent front portion allowing light entry.
Limbus — Junction between cornea and sclera housing stem cells for corneal epithelium.
Choroid — Vascular layer supplying blood to the eye.
Ciliary body — Thickened region producing aqueous humor and controlling lens shape.
Iris — Colored diaphragm regulating pupil size.
Pigment layer — Outer retinal layer rich in melanin.
Neural layer — Inner retinal layer containing rods and cones.
Vitreous humor — Gel filling main eye chamber.
Aqueous humor — Fluid in anterior eye chamber, maintains pressure.