Table : Differences between rod cells and cone cells
Rod cells Cone cells- rod cells are far more numerous than cone cells,
about 125 millions in human eye.
cone cells are far fewer than rod cells, only about 7 millions
in human eye.- Long, slender and filamentous. Shorter and thicker.
- Outer segment is cylindrical and contains rhodopsin
pigment.
Outer segment is conical and contains iodopsin pigment.- Inner end has a small knob. Inner end is branched.
- Sensitive to dim light, and give “twilight” vision. Sensitive to bright light, and give “daylight” vision.
- Insufficient rhodopsin results in night blindness. Insufficient iodopsin results in colour blindness.
- all rod cells are alike, and do not give colour vision. cones are of 3 types: blue, green and red, and gives colour
vision.
• Light changes the cone pigment chemically and this sets
up nerve impulse.
• Lack of one or more types of cone cells causes colour
blindness.
Blind spot (also called optic disc)
• It is an area of retina from where the optic nerve leaves
the eye.
• It does not have any rods and cones, and so is not light
sensitive.
• No image is formed at the blind spot.
Optic nerve
• It contains the fibres of the sensory neurons and leaves
the eye ball from the back side.
• It carries visual impulses from the retina to the brain.
Macula lutea (Yellow spot)
• a small area of optical part of the retina lying exactly
opposite to the centre of the cornea is called the
macula lutea or yellow spot.
• It is about 5 mm in diameter and has a yellow pigment
(xanthophyll) which probably screens out some of
blue part of the spectrum to protect the photoreceptors
from blinding effect of bright light.
Fovea centralis
• Fovea centralis is a shallow depression present in the
middle of macula lutea. The centre of the fovea is called
‘foveola’.
• lt lacks blood vessels and has no layer of nerve fibres.
The eye’s sharpest and most brilliantly coloured vision
occurs when light is focused on the fovea centralis.
• This region exclusively has cones (some around
30,000) and they are smaller and more closely packed
than elsewhere on the retina.
• These cone cells are attached to individual neurons
which provide high resolution colour image.
• From the fovea to the periphery, cones diminish and
rods increase in number. hence, in the periphery, only
twilight vision occur, and no colour vision.
• Fovea centralis is the most sensitive part of retina.
Because the fovea has no rods, small dim objects in
the dark cannot be seen if one looks directly at them.
• For this reason, to detect faint stars in the sky, one must
look just to the side of them, so that their light falls on
a retinal area, containing numerous rods, outside the
macular zone.Contents of the eye ball
Lens
• It is a transparent, elastic and a biconvex structure held
in position by suspensory ligaments, which extend
from the equatorial edge of the lens to the ciliary
processes.
• Lens is made up of non-nucleated, transparent and
elongated cells having elastin protein.
• It undergoes a change in focal length due to the stretching
and relaxation of the suspensory ligaments and brings
the adjustment for focusing of light on retina.
• The lens and suspensory ligament divide the cavity
of the eyeball into two chambers: the anterior small
aqueous chamber and the posterior large vitreous
chamber.
(i) Aqueous chamber
• The aqueous chamber itself consists of two regions