What is the physiology of the eye?

The eye is composed of a series of lenses and spaces that give focus to images, just as a camera does. It is composed of the vitreous humor, aqueous humor, the crystalline lens, and the cornea, and each of these has its own refraction index (the average being 1.34, because of the content of these tissues).

Physiology of the Eye

The primary function of the eye is to form a clear image of objects in our environment. These images are transmitted to the brain through the optic nerve and the posterior visual pathways.

The various tissues of the eye and its adnexa are thus designed to facilitate this function.

The Eyelids

Functions include: (1) protection of the eye from mechanical trauma, extremes of temperature and bright light, and (2) maintenance of the normal precorneal tear film, which is important for maintenance of corneal health and clarity.

The Tear Film

The tear film consists of three layers: the mucoid, aqueous and oily layers.

The mucoid layer lies adjacent to the corneal epithelium.It improves the wetting properties of the tears.

The Cornea

The primary function of the cornea is refraction. In order to perform this function, the cornea requires the following:

• transparency
• smooth and regular surface
• spherical curvature of proper refractive power
• appropriate index of refraction.

The Aqueous Humour

The aqueous humour is an optically clear solution of electrolytes (in water) that fills the space between the cornea and the lens. Normal volume is 0.3 ml. Its function is to nourish the lens and cornea.

The Vitreous Body

The vitreous consists of a three-dimensional network of collagen fibers with the interspaces filled with polymerized hyaluronic acid molecules, which are capable of holding large quantities of water.

The Lens

The lens, like the cornea, is transparent. It is avascular and depends on the aqueous for nourishment.

It has a thick elastic capsule, which prevents molecules (e.g., proteins) moving into or out of it.

The Ciliary Body

The ciliary muscle (within the ciliary body) is a mass of smooth muscle, which runs circumferentially inside the globe and is attached to the scleral spur anteriorly.

Accommodation

Accommodation is the process whereby relaxation of zonular fibers allows the lens to become more globular, thereby increasing its refractive power.

The Retina

This is the “photographic film” of the eye that converts light into electrical energy (transduction) for transmission to the brain.