10. The Eye
Sight organ with three layers: sclera (tough outer coat, continues as the cornea), choroid (middle, pigmented, blood vessels), retina (innermost, contains rods and cones).
|
Structure |
Function |
|
Cornea |
Refracts light most strongly; protects the eye |
|
Pupil / Iris |
Iris muscles control pupil size to regulate light entry |
|
Lens |
Fine-focuses light onto the retina (accommodation) |
|
Aqueous / Vitreous humour |
Refract light; maintain eyeball shape; supply nutrients |
|
Retina (Rods) |
Dim-light (night) vision; no colour; low acuity |
|
Retina (Cones) |
Bright-light/colour vision; high acuity; concentrated at the fovea |
|
Fovea |
Region of sharpest vision (cones only) |
|
Blind spot |
Optic nerve exit point; no photoreceptors |
Accommodation
- Distant object: circular ciliary muscles relax, radial muscles contract, ligaments taut, lens thin/flat — less refraction.
- Near object: circular ciliary muscles contract, ligaments slack, lens thick/convex — more refraction.
Common Eye Defects
|
Defect |
Cause |
Correction |
|
Myopia (short sight) |
Eyeball too long / cornea too curved — image focuses in front of retina |
Concave (diverging) lens |
|
Hypermetropia (long sight) |
Eyeball too short — image focuses behind retina |
Convex (converging) lens |
|
Astigmatism |
Uneven curvature of the cornea |
Cylindrical lens |
|
Cataract |
Lens becomes opaque (age, UV, smoking) |
Surgery |
|
Glaucoma |
Raised intra-ocular pressure damages the optic nerve |
Laser/surgery, eye drops |
- Colour blindness: caused by absent/defective cones; commonly a sex-linked recessive inherited condition, more frequent in males.