Lens, Vitreous Body, Development

 The lens:

• It is transparent
• It is a biconvex structure.
• It is placed between the anterior and posterior segments of the eyeball.
• It is circular in outline
• It has a diameter of 1cm
• The central points of anterior and posterior surfaces are known as anterior and posterior poles.
• The line connecting the poles constitutes axis of lens
• The marginal circumference is termed as equator

• The chief advantage of the lens is that it can vary its dioptric power.
• It contributes about 15 diopters to the total of 58 dioptric power of the eye.
• A diopter is the inverse of the focal lengths in meters.
• A lens having a focal length of half a meter has a power of 2 diopters.

• The posterior surface of the lens is more convex than the anterior.
• The anterior surface is kept flattened by the tension of suspensory ligaments
• When the ligament is relaxed by the contraction of the ciliary muscles, the anterior surface becomes more convex due to the elasticity of the lens substance.

• The lens is enclosed in a transparent, structureless elastic capsule.
• The capsule is thickest anteriorly near the circumference.
• Deep to the capsule, the anterior surface of the lens is covered by a capsular epithelium.
• At the centre of the anterior surface, the epithelium is made up of a single layer of cuboidal cells.
• At the periphery, the cells elongate to produce the fibres of the lens.
• The fibres are concentrically arranged to form the lens substance.
• The centre of the lens is the nucleus of the lens, which is firm.
• The periphery is the cortex of the lens, which is soft and is made up of more recently formed fibres.

• The suspensory ligament of the lens or zonule of Zinn retains the lens in position
• Its tension keeps the anterior surface of the lens flattened.
• The ligament is made up of a series of fibres which are attached peripherally to the ciliary process, to the furrows between the ciliary processes and to the ora serrata.
• Centrally the fibres are attached to the lens, mostly in front and few behind the equator.

Clinical Anatomy:

• Lens become opaque with increasing age - Cataract. Since the opacities cause difficulty I vision. Here the lens has to be replaced
• The central artery of retina is an end artery. Blockage of the artery leads to sudden blindness.
• Third nerve paralysis causes partial ptosis and dilated pupil. The cornea is turned downwards and outwards.
• Horner’s syndrome results in partial ptosis and miosis
• In brainstem death, both pupils are dilated and fixed.
• Eye sees everyone. One can see the interior of the eye by ophthalmoscope. Through the ophthalmoscope, one can see the small vessels in the retina and judges the changes in diabetes and hypertension. In addition, one can also examine the optic disc for evidence of papilloedema, caused by raised intracranial pressure.

Vitreous Body:

• It is colourless
• It is a jelly-like transparent mass which fills the posterior segment of the eyeball (posterior 4/5th of eyeball)
• It is enclosed in a delicate homogenous hyaloid membrane. 
• Behind it is attached to the optic disc and in front to ora serrata.
• In between it is free and lies in contact with the retina.
• The anterior surface of the vitreous body is indented by the lens and ciliary processes

Development:

• Optic vesicle forms the optic cup.
• It is an out pouching from the forebrain vesicle.
• Lens from lens placode ( ectodermal )
• Retina  - Pigment layer  from the outer layer of optic cup.
• Nervous layer from the inner layer of optic cup.
• Coroid, sclera from mesoderm.
• Cornea from the surface ectoderm forms the epithelium, other layers develop from mesoderm.