- 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.
- 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.
- 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.
- 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:
- 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
- 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.
- 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
- 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.
- 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
- 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.
- 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.