THE NORMAL EYE

The human eye is round like a ball about one inch or 2.5 cm diameter it is held in a bony socket (orbital cavity) by muscles and connected in the brain by a nerve called OPTIC NERVE. It is covered in the front by the upper and lower eye lids, which must open and close properly. The eyeball is divided into two segments anterior and posterior chamber which are filled with fluids called aqueous humor in anterior chamber while vitreous humor in the posterior chamber. The fluids are responsible for maintaining shape and pressure. 

The human eye is roughly spherical in shape. It is bounded by three distinct layers of tissue. The outer layer, the Sclera/sclerotic coat, is extremely tough. It is white in colour (the “white" of the eye) except in the front. Here it forms the transparent cornea, which admits light into the interior of the eye and bends the light rays so that they can be brought to a focus. The surface of the cornea is kept moist and dust-free by the secretion from the tear glands.

The eye ball consists of three (3) major layers:

• A protective layer: Cornea and Sclera.

• A middle vascular layer: (Uvea) Iris, Ciliary body and Choroid.

• An inner visual layer: Retina and optic nerve.

The four (4) Section of the eye

1. The protective coat consist of:

   1.  Sclera

   2. Cornea

   3. Aqueous 

   4. Lens

2. Vascular layer consist of:

1. Iris

2. Ciliary body

3. Choroid

3. Visual layer :

   1. Retina 

   2. Optic nerve

4. Other contents are:

   1. Aqueous humor

   2. Vitreous body  

Outer part of the eye and its functions 

   Outer part 

• Eyebrow which prevents the sweat from the forehead 

• Eyelids which cover the eyes when not in use 

• Eyelashes which catch dirt to prevent it from getting into the eyes 

Visible part 

• Cornea is the outer layer of an eye.  It is thin and transparent. 

• Iris lies behind the cornea and it is the coloured part of the eye 

• Pupil controls the amount of light that enters the eye. It is black and round. 

• Aqueous humor is a thick liquid that nourishes the bloodless, transparent cornea and the lens  

Inner part 

• Vitreous humor is a thick substance between the lens and the retina 

• Retina is the innermost layer of the eyeball situated behind the vitreous humor. 

• Optic nerve is the nerve cell that joins at the back of the retina. It sends message to the brain.  

Diagram: Draw the cross section of the normal eye

Simple description of the eye layers

The Cornea: Is the clear cover in the front of the eye. It covers the colored part of the eye, called the iris. The cornea focuses light into the eye.

The Sclera: This is the outer layer and is about one millimeter thick; it is composed by white fibrous tissue given strength and support the eye ball from the front part.

The Conjunctivae: The conjunctivae is a clear or transparent membrane covering the front of the sclera and folds over to line the inner part of the eye lids.

The Iris: Iris is attached to the central fringe of the ciliary muscles and is the colored part of the eye; it forms a ciliary screen in front of the lens and is responsible for controlling the amount of light entering the eye.

The Choroid: This is very thin layer, situated at middle coat of the eye; consist of dense net-work of capillaries. It extends to the margin of the cornea, where it is joined to the ciliary body. 

The Ciliary body: Is mainly muscular and circular consisting of 2 sets of fibres on in front of the other. The posterior set is called the ciliary body is responsible for changing the thickness of the lens so that light rays are to near vision and vis – a – vis.

The Pupil:  This refers to the black round portion in the center of the iris, which controls the amount of light entering the eye. In dim or dark areas the pupil dilates to let more light into the eye, while in the bright light areas, the pupil constricts to let in less light (responsible for adequate and proper light accommodation).

Anterior Chamber: Is located behind the cornea but in front of lens. It contains the clear fluids called aqueous fluids, which escapes from the eye through small openings at the base of the iris and cornea called the angle of the anterior chamber. This really helps to control the pressure inside the eye.

The Lens: Is suspended behind the iris and pupil by the suspensory ligament attached to the ciliary body. The lens focuses the light onto the retina.

Vitreous humor: The vitreous is a clear thick jelly – like liquid that fills the space between the lens and the retina. Its help to give the eye ball round shape.

The Retina: The retina is the inner lining of the back portion of the eye ball; it can be seen with the aid of a special instrument e.g ophthalmoscope. Retina contains the rods, cones and nerve. The rod and cones respond to light and color. The rods helps individual to see at night while the cones are essential for seeing things in details and also for seen colors. The carry sensation of sight (an information of what eye sees) from the retina to brain. They leave the eye in the optic nerve which connects the eye to the brain.

Eye lids and lacrimal system: The eyelids have the eyelashes that are directed away from the globe. The eyelids have special muscles (orbicularies) for closure of the eyes and (levator palpebrae superioris) for opening of the eyes, they cover the conjunctiva and with each blink clean dust and other particles from the eye and moisten it with tears.

The eyelashes keep small objects from getting into the eye by trapping them. On the medial side of each eyelid is a tiny opening called the punctum, the tears produced by the tear gland enter into the punctum and drain through the tear passages.

Extrinsic muscles of the eye

For the human eye to function and move correctly, six muscles must work in unison to provide three-dimensional vision. The six muscles of the eye are: 

• Superior rectus 

• Medial rectus  

• Lateral rectus  

• Inferior rectus  

• Superior oblique  

• Inferior oblique