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 skull and is supplied by optic nerve.
- There are 6 sets of muscles attached to outer surface of eye ball which helps to rotate it in 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 functions as a pair.
Anatomical structure of Eye
Eye ball 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 following parts.
- It is outermost supporting layer consists of thick membrane of tough fibrous connective tissue.
- It covers 5/6 parts of eye ball.
- It maintains the shape of eye and provide attachment to the extrinsic muscle of eye
- It is a thin transparent front part of sclera.
- It forms a slight bulge at the front and covers an anterior 1/6 part of sclera.
- Cornea is avascular and absorbs oxygen from air.
- It refracts light to focus on retina.
- It is a thin transparent layer that covers the cornea.
- It is formed of single layer of stratified squamous epithelium
- It protects the cornea.
II. Middle vascular layer:
It consists of following parts:
- It is thick vascular and pigmented layer situated below sclera.
- The pigmented cells absorbs light and prevent it from being reflected.
- The function of 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 ciliary muscle and suspensory ligament.
- Ciliary body is attached to lens and holds it in position
- Its function is to change the shape of lens by contraction or relaxation of muscle
- It is muscular, pigmented and opaque diaphragm which hangs in the eye ball in front of lens.
- It has small circular opening called pupil.
- It has two types of muscles; circular and radial muscle. The movement of these muscles control the size of pupil.
- Pigment in iris gives color to eye.
- Iris control the amount of light entering into eye by controlling the size of pupil.
III. Inner layer:
It consists of photoreceptor cells and photo sensitive elements.
- Retina is innermost layer.
- Neuroretina contains highly specialized photoreceptor nerve cells; rods and cones
- Each eye ball has 125 millions of rod cells and 7 millions of cone cells.
- Small depression in retinal wall is called Fovea centralis which contains only cone cells.
- Fovea centralis is highly sensitive to light and forms magnified image and give sharp and acute vision.
- The optic nerve enter retina at a point called blind spot. It does not contains any rods or cone cells. It is least sensitive to light and forms no image when light falls on blind spot
- rods are sensors for perception of black to white shades
- Night vision is almost rod vision.
- It function in dim light
- Contains a photosensitive pigment rhodopsin formed from vitamin A.
- Cones are sensors for perception of colors.
- It functions in bright light and differentiate 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.
- Lens is enclosed in a transparent elastic capsule.
- Ciliary muscles control the thickness of lens and its power of accommodation.
- It forms the image of the object on retina.
- Lens separates the eye ball into two chamber
i. Aqueous chamber
ii. Vitreous chamber
- It is a smaller fluid filled chamber between cornea and lens.
- It is filled with aqueous humour containing aminoacids, glucose, ascorbic acid, hyaluronic acid and respiratory gases.
- The aqueous humour nourishes the lens and cornea and refracts light rays to focus on retina.
- It is a larger fluid filled chamber between lens and retina.
- It is filled with gelatinous vitreous humour containing salts and muco proteins.
- It supports retina and refracts light to focus on retina.