Eye

Human eye is a special sense organ responsible for the sense of sight. Structurally it is in the form of a sphere (the eyeball) that is contained within the orbital cavity of the skull.

Structure of the eye:

Within the orbit, the eyeball is embedded in the orbital fat, separated from it by the facial sheath of the eyeball. The eyeball itself consists of three coats, enumerated below (in order from exterior to interior);

  • Fibrous coat
  • Vascular pigmented coat
  • Nervous coat

Inside the coats lie the contents of the eyeball, which include;

  • Refractive media
  • Aqueous humor
  • Vitreous body
  • Lens

Coats of the eyeball:

Fibrous coat:

The fibrous coat of the eyeball is made up of an opaque posterior part called sclera and a transparent anterior part called cornea.

Sclera:

It is the opaque posterior part of the fibrous coat of the eyeball. It is made up of dense fibrous tissue and is white in color.

In its posterior-most portion, the sclera is pierced by the optic nerve. This portion of the sclera is called lamina cribrosa. It is here that the sclera fuses with the dural sheath of the optic nerve.

The sclera is also pierced by other structures including:

  • Ciliary nerves
  • Ciliary arteries and their associated veins
  • Vorticose veins

Anteriorly, the sclera is continuous with the cornea at the corneoscleral junction, also knows as limbus.

Cornea:

Cornea is the transparent, anterior-most part of the fibrous coat of eyeball. It is largely responsible for the refraction of light entering the eye.

The cornea is in direct contact with the aqueous humor posteriorly. Since the cornea is avascular and devoid of lymph vessels, the nourishment is provided by diffusion from the aqueous humor and from capillaries at the edge. The tear film on the anterior surface of the cornea is very important for maintaining a balanced environment for the corneal epithelial cells.

  • Nerve Supply: It is innervated by the long ciliary nerves (branches of ophthalmic division of trigeminal nerve).
  • Functions: Cornea is the most important refractory medium of the eye. This is because the refractive index of cornea (1.38) and that of air (1.0) differ greatly.

Vascular pigmented coat:

It is the second coat of the eyeball, lying inside the fibrous coat. It consists of three parts, which from posterior to anterior are; the choroid, the ciliary body, the iris.

The choroid:

It is the vascular layer of the eye that is composed of an outer pigmented layer and an inner highly vascular layer. It provides oxygen and nourishment to the outer layer of retina.

The Ciliary body:

Ciliary body is continuous posteriorly with the choroid. Anteriorly, it lies behind the peripheral margin of the iris. It is composed of three components; ciliary ring, ciliary processes and ciliary muscle.

  • Ciliary ring: It is the posterior part of the ciliary body.
  • Ciliary processes: These are folds, or ridges that are radially arranged. They are connected to the suspensory ligaments of the lens.
  • Ciliary muscle: It is a ring-shaped muscle, composed of longitudinal and circular fibers of smooth muscle. It controls the accommodation by modifying the size of the lens.

The Iris:

Iris is a thin, contractile, pigmented diaphragm that has a central aperture, the pupil. Peripherally, the iris is attached to the ciliary body. The color of the iris is generally referred to as the “eye color”.

The muscle fibers of the iris are involuntary and consist of circular and radial fibers. The circular fibers form the sphincter pupillae and the radial fibers form the dilator pupillae. These muscles control the aperture of the pupil.

Nervous coat: The retina

The third and innermost coat of the eyeball is the nervous coat, called retina. It consists of two layers; outer pigmented layer (that is in contact with the choroid) and inner nervous layer (that is in contact with the vitreous body).

Contents of the eyeball:

Contents of the eyeball are refractive media, aqueous humor, vitreous body and lens.

Aqueous Humor:

It is a clear fluid that fills the anterior and posterior chambers of the eyeball. It is secreted from the ciliary processes into the posterior chamber, from where it enters the anterior chamber through the pupil. It is then drained away through the spaces at the iridocorneal angle into the canal of schlemn.

If draining of aqueous humor is obstructed, it results in a rise in intra-ocular pressure called glaucoma.

The main function of aqueous humor is to support the wall of the eyeball and maintaining its optical shape by exerting an internal pressure. Furthermore, it also nourishes the cornea and the lens and removes the products of metabolism. Considering the fact that cornea and lens has no blood supply, the second function is also very important.

Vitreous Body:

It is a transparent gel that fills the eyeball behind the lens. There is a narrow channel that runs through the vitreous body from the optic disk to the posterior surface of the lens and is called the hyaloid canal. In the fetus, the hyaloid canal contains the hyaloid artery, which disappears before birth.

The function of the vitreous body is the contribute to the magnifying power of the eye. It also holds the posterior surface of the lens and assists in holding the neural part of the retina against the pigmented part.

Lens:

It is a transparent, biconvex structure that is enclosed in a transparent capsule. It lies behind the iris and in front of the vitreous body. The ciliary body encircles it.

Movements:

The movements of the eyeball are described in relation to the direction of the movement of the anterior pole of the eye, which is the center of the cornea or center of the pupil. The movements are:

  • Elevation: Upward rotation of the eyeball i.e. towards the forehead
  • Depression: Downward rotation of the eyeball i.e. towards the cheeks
  • Abduction: Lateral rotation of the eyeball i.e. towards the ears
  • Adduction: Medial rotation of the eyeball i.e. towards the nose

Extrinsic muscles:

Muscle

Origin

Insertion

Nerve Supply

Action

Superior Rectus

Annulus of Zinn

Superior surface of eyeball

Oculomoter Nerve (Cranial Nerve III)

Raises cornea upward and medially

Inferior Rectus

Annulus of Zinn

Inferior surface of eyeball

Oculomoter Nerve (Cranial Nerve III)

Depresses cornea downward and medially

Medial Rectus

Annulus of Zinn

Medial surface of eyeball

Oculomoter Nerve (Cranial Nerve III)

Rotates eyeball so that corneal looks medially

Lateral Rectus

Annulus of Zinn

Lateral surface of eyeball

Abducent nerve (Cranial nerve VI)

Rotates eyeball so that cornea looks laterally

Superior Oblique

Posterior wall of orbital cavity

Passes through the pulley and is attatched to superior surface of eyeball just beneath the superior rectus

Trochlear nerve (Cranial nerve IV)

Rotates eyeball so that cornea looks downward and laterally

Inferior Oblique

Floor of orbital cavity

Lateral surface of eyeball deep to the lateral rectus

Oculomoter nerve (Cranial nerve III)

Rotates eyeball so that cornea looks upward and laterally

Intrinsic muscles:

These are involuntary muscles that are concerned with the accommodation and light reflexes. They take no part in movements of the eyeball.

Muscle Nerve Supply Action
Sphincter Pupillae of iris Parasympathetic (via oculomotor nerve) Constricts the pupil
Dilator Pupillae of iris Sympathetic Dilates the pupil
Ciliary muscle Parasympathetic (via oculomotor nerve) Controls the shape of the lens hence is responsible for accommodation