EYE

   EYE

1.INTRODUCTION:

       The eye is the organ of the sense of sight or vision .

2.SITUATION:

It  is situated in the orbital cavity.

3.MEASURMENTS:

SHAPE:          -             Spherical
 DIAMETER          -        2.5cm

v  The space between eye and the orbital cavity is occupied by adipose tissue.

v  The bony walls of the orbit and the fat help to protect the from injury.

4.ACCESSORY  ORGANS OF THE EYE:

The eye is a delicate organ which is protected by several structures.

     Eye brows

  Eye lids and eye lashes

Lacrimial apparatus.

5.STRUCTURE:

 There are 3 layers of tissue in the walls of the eye

1.The outer fibrous layer : sclera & cornea

2.the middle vascular layer (or) uveal tract :choroid ciliary body & iris.

3. the inner nervous tissue layer : retina.

4.structures inside the eyeball are the lens ,aqueous fluid (humour) & vitreous  body (humour).

SCLERA & CORNEA:

THE SCLERA

1.  It is a white part of the eye.
2. It forms the outermost layer of tissue of the posterior and lateral aspects of the eye ball,is continous anteriorly with the transparent cornea.
3. It consists of a firm fibrous membrane that maintains the shape of the eye and gives attachment to the extraocular (or) extrinsic muscles of the eye.
4. Anteriorly the sclera continues as a clear transparent epithelial membrane, the ‘cornea’.
5. Light rays pass through the cornea to reach the retina.
6. The cornea is convex anteriorly and is involved in refracting or bending light rays to focus them on the retina.

CHOROID:

1. The choroid lines the posteriorly five-sixth of the inner surface of the sclera.
2. It is very rich in blood vessels and is deep chocolate brown in colour.
3. Light enters the eye through the pupil , stimulates the nerve endings in the retina and is then absorbed by the choroid.

CILIARY BODY:

1. The ciliary body is the anterior contituation of the choroid consisting of ciliary muscle and secretory epithelial cells.
2. It gives attachment to the sensory ligament which at its other end is attached to the capsule enclosing the lens.
3. Contraction and relaxation of the ciliary muscle changes the thickness of the lens which bends or refracts light rays entering the to focus them on the retina.
4. The epithelial cells  secrete “Aqueos fluid “ into the anterior segment of the eye,I,e the space between the lens and the cornea (anterior & posterior compartment.
5. The ciliary body is supplied by parasymphathetic branches of the oculo motor nerve(3^rd cranial nerve).
6. Stimulation causes contraction of the smooth muscle and accommodation of the eye.

IRIS:

1. The iris is the visible coloured part of the eye & extends anteriorly from the ciliary body,lying behind the cornea infront of the lens.
2. The colour of the iris is genetically determined and depends on the number of pigment cells present.
3. Albinos have no pigment cells and people with blue eyes have fewer then those with brown eyes.
4. It divides the anterior segment of the eye into anterior and posterior chambers which contains “Aqueous fluid” secreted by the ciliary body.
5. It is a circular body composed of pigment cells and two layers of smooth muscle fibres, one circular and other radiating.
6. In the centre there is an aperture called the PUPIL.
7. The iris is supplied by parasymphathetic and parasymphethetic nerves.
8. Parasympathetic nerve stimulation constricts the pupil and sympathetic stimulation dilates the pupil.

   LENS

1. The lens is a highly elastic circular biconvex body,lying immediately behind the pupil.
2. It consists of fibres enclosed within a capsule and it is suspended from the ciliary body by the suspensory ligament .
3. The lens bends (refracts) light rays relected by objects in front of the eye.
4. Lens is the structure in the eye that can vary its refractory power,achived by changing its thickness.
5. When the ciliary muscle contracts, it moves forward,releasing its pull on the lens, increasing its thickness.
6. The nearer is the object being  viewed the thicker the lens becomes to allow focusing.

RETINA:

1. The retina is the innermost layer of the wall of the eye.
2. It is an extremely delicate structure and is especially adapted for stimulation by light rays.
3. It is composed of several layers of nerve cell bodies and their axons, lying as a pigmented layer of epithelial cells which attach it to the choroid.
4. The layer highly sensitive to light is the layer of sensory receptor cells :rods & cones.
5. The retina lines about three-quarters of the eyeball.
6. The eyeball is thickest at the back and things out anteriorly to end just behind the ciliary body.
7. Near the center of the posterior part is the  macculalutea, (or) yellow spot.
8. In the centre of the area there is a little depression called “fovea centralis”. Consisting of only ‘cone-shaped cells.
9. The rods and cones contain photosensitive pigments that convert light rays into nerve impulses.
10. About 0.5 cm to the nasal side of the macula lutea all the nerve fibres of the retina converge to form the optic nerve.
11. The small area of the retina where the optic nerve leaves the eye is the optic disc (or) blind spot.
12. It has no light sensitive cells.

6.BLOOD SUPPLY:

 ARTERIAL BLOOD SUPPLY:

•  Cliary artery and thr central retinal artery these are the branches of ophthalmic artery and branches  of the carotid artery.

VENOUS DRAINAGE:

• Central retinal vein which eventually empty into a deep venous  sinus. 

Human Body Structure - click here