b.What is Lens
Lens is colourless ,transparent and fibrous crystalline structure made up of protein (alpha and bita crystalline protein)and enclosed in lens membrane.It is ectodermal in origin. Lens is lodged in eyeball by suspensory ligament of ciliary body. Suspensory ligaments are known as "Zonula of Zinn" . In man lens is biconvex while in frog it is elliptical (subspherical).
Lens divide the eyeball in 2 chamber outer aqueous chamber(partially divided into a large anterior and a smaller posterior chamber) filled with aqueous humor(watery) formed by ciliary body and inner vitreous chamber filled with vitreous jelly (or Wharton's jelly) containing 99% water,some salt a little mucoprotein(vitrein) and hyaluronic acid.
Difference between Aqueous humour and Vitreous humour
|S.No||Aqueous humour||Vitreous humour|
|1||It occurs in aqueous chamber.||It occurs in vitreous chamber|
|2.||It is a watery fluid||It is a jelly-like substance.|
|3.||It is secreted by ciliary processes||It is apparently secreted by retina during developement of eye.|
|4||It is continuously absorbed into blood and replaced.||It is not absorbed or replaced.|
|5||It contains most of the diffusibe substance of the plasma||It consits of water(90%) protein vitrein,hyaluronic acid and collagen fibres.|
|6.||Obstruction in its flow may damage retina by increasing intraocular pressure.||It does not flow|
|7.||Refraction index is 1.33 D||Refraction index is 1.34 D|
RetinaIt is innermost ,thin and transparent ,purplish red due to the presence of the eye pigment rhodopsin(in rods) or visual purple which was extracted by Kuhne(1876) and name "Schpurpur"(visual purple).Made up of 4 distinct layer__
a. Cuboidal pigmented epithelium(towards choroid).
b. Layer of rods and cones.
c. Layer of bipolar neurons.
d. Layer of ganglia (Towards vitreous chamber innermost).
Area centralis of retina
A little part of retina that lies upon the optical axis is called area centralis. Here ,the retina is very thin and contains only cone cell filled with a yellow pigment. Hence ,this part is called yellow spot or macula lutea. In man (Rabbits) and other mammals, but not in frog,this area has a small shallow depression called fovea centralis.This later is the most sensitive part of an eye.ie. the area of most acute vision.It is also claimed that the cone cells in fovea centralis are placed somewhat obliquely.So that these can from magnified images of object.
Blind spot(Optic disc)
At this point,the optic nerve turns towards the outer side,pierces through the whole thickness of the wall of eyeball, forming an optic foramen and runs to the brain. Obviously ,the region of optic foramen has no retina.It therefore, does not take part in image formation and is called blind spot.
Difference between Blind spot and Yellow spot
It lies a little away from the yellow spot.It contains no pigment. Optic nerve starts from this spot.It lacks a depression.It lacks visual receptors and is insensitive to light.The eye coats are absent at blind spot. No image is formed at this place.
It lies exactly opposite the centre of the cornea.It has a yellow pigment.No nerve starts from this spot.It has a shallow depression,the fovea centralis,at its middle. It has visual receptors and is sensitive to light. Eye coats are present at yellow spot.Image is formed at this place.