Human anatomy

Human Eye: Anatomy, parts and structure

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  • 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.
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