About Lesson
Eyes are organ of sight which is able to perceive the radiant energy of visible light and can convert it into nerve impulse.
Eyeball is formed of following parts:
a. Fibrous tunic
- Outermost thick layer of eyeball
Formed of following parts
i. Sclera
- Non pigmented outermost covering
- Maintains shape of eyeball
- Form 5/6 part of eyeball
- Small portion is visible from outside called ‘white of eye’
ii. Cornea
- Form 1/6th part of fibrous tunic
- Transparent and avascular
- Converges the light to focus on retina.
iii. Conjunctiva
- Visible part of eye and cornea
- Meant for protection of cornea
b. Pigmented coat
It is middle and incomplete layer. Formed of following parts:
i. Choroid
- Darkly pigmented vascular layer present just beneath the sclerotic.
- Absorbs the light and prevent it from being refracted.
ii. Cilliary body
- Pigmented part present at junction of sclerotic and cornea
- Gave rise to suspensory ligament which holds the lens
- Contains cilliary muscle which contraction can change thickness of lens
- Lens plays important role to focus the image on retina.
iii. Iris
- Pigmented, provide color to eye
- Centrally perforated through which light enters into eyeball called Pupil.
- Determines amount of light entering into eyeball.
c. Retina
- Innermost, pigmented, most sensitive and incomplete layer.
- Formed of four cells. They are: Pigmented layer, Photosensitive layer, Bipolar layer and Ganglionic layer.
- Contains rods and cones cells
- Rods cells are numerous and sensitive to dim light
- Cones cells are less in number and sensitive to bright light.