Human Eye: Anatomy, parts and structure

• The eye is the photo-receptor organ.
• Size and shape: Human eye is spherical about 2.5 cm in diameter.
• Location: it is situated on an orbit of the skull and is supplied by the optic nerve.
• There are 6 sets of muscles attached to the outer surface of the eyeball which helps to rotate it in a different direction.
• Four sets of these muscles are straight muscles; superior, inferior, medial and lateral rectal muscle and two sets are oblique muscles; superior and inferior oblique muscles.
• Structurally two eyes are separated but some of their activities are coordinated so that they function as a pair.

The anatomical structure of Eye

Eyeball consists of three layers

• Outer fibrous layer: Sclera, cornea, and conjunctiva
• Middle vascular layer: ciliary body, choroid, and iris
• Inner layer: retina

I. Outer fibrous layer:
It consists of the following parts.
1. Sclera:

• It is outermost supporting layer consists of a thick membrane of tough fibrous connective tissue.
• It covers 5/6 parts of the eyeball.
• It maintains the shape of the eye and provides attachment to the extrinsic muscle of the eye.

2. Cornea:

• It is a thin transparent front part of the sclera.
• It forms a slight bulge at the front and covers an anterior 1/6 part of the sclera.
• The cornea is avascular and absorbs oxygen from the air.
• It refracts light to focus on the retina.

3. Conjunctiva:

• It is a thin transparent layer that covers the cornea.
• It is formed of a single layer of stratified squamous epithelium
• It protects the cornea.

II. Middle vascular layer:
It consists of the following parts:
1. Choroid:

• It is a thick vascular and pigmented layer situated below sclera.
• The pigmented cells absorb light and prevent it from being reflected.
• The function of the choroid is to provide nutrition and to prevent reflection of light.

2. Ciliary body:

• These are attached to choroid and present at the junction of sclera and cornea.
• It consists of two sets of the ciliary muscle and suspensory ligament.
• The ciliary body is attached to the lens and holds it in position
• Its function is to change the shape of the lens by contraction or relaxation of muscle

3. Iris:

• It is muscular, pigmented and opaque diaphragm which hangs in the eyeball in front of the lens.
• It has a small circular opening called pupil.
• It has two types of muscles; circular and radial muscle. The movement of these muscles controls the size of the pupil.
• The pigment in the iris gives color to the eye.
• Iris controls the amount of light entering into eye by controlling the size of the pupil.

III. Inner layer:
It consists of photoreceptor cells and photosensitive elements.
1. Retina:

• The retina is innermost layer.
• Neuroretina contains highly specialized photoreceptor nerve cells; rods and cones
• Each eyeball has 125 millions of rod cells and 7 millions of cone cells.
• A small depression in the retinal wall is called Fovea centralis which contains only cone cells.
• Fovea centralis is highly sensitive to light and forms a magnified image and give sharp and acute vision.
• The optic nerve enter retina at a point called a blind spot. It does not contain any rods or cone cells. It is least sensitive to light and forms no image when light falls on a blind spot

Rod cell:

• rods are sensors for the perception of black to white shades
• Night vision is almost rod vision.
• Its functions in dim light
• Contains photosensitive pigment rhodopsin formed from vitamin A.

Cone cell:

• Cones are sensors for perception of colors.
• It functions in bright light and differentiates colors.
• Contains a photosensitive pigment iodopsin.

Eye lens and chambers

1. Eye Lens:

• It is a large, flexible, transparent biconvex and fibrous crystalline body situated behind iris.
• The lens is enclosed in a transparent elastic capsule.
• Ciliary muscles control the thickness of the lens and its power of accommodation.
• It forms the image of the object on the retina.
• Lens separates the eyeball into two chambers

i. Aqueous chamber
ii. Vitreous chamber
Aqueous chamber:

• It is a smaller fluid-filled chamber between cornea and lens.
• It is filled with aqueous humor containing amino acids, glucose, ascorbic acid, hyaluronic acid, and respiratory gases.
• The aqueous humor nourishes the lens and cornea and refracts light rays to focus on the retina.

Vitreous chamber:

• It is a larger fluid-filled chamber between lens and retina.
• It is filled with gelatinous vitreous humor containing salts and mucoproteins.
• It supports retina and refracts light to focus on the retina.