14.2) Sense organs

14.2) Sense organs

Sense organs: are groups of sensory cells responding to specific stimuli, such as light, sound, touch, temperature and chemicals.

Pupil reflex:

  • The pupil of the eye is the dark round area in the centre of it. It is surrounded by a coloured ring structure called the iris.
  • The pupil and ciliary muscle together play a big role in protecting the eye from damage by limiting the amount of light entering the eye. If too much light fall on the retina, the rods and cones get damaged. The iris and pupil change their size to smiddle that happening.
  • The iris contains two sets of muscles; Circular and Radial muscles. Circular muscles run around the iris and radial muscles run from the centre to the outside.
  • In bright light circular muscles contract to make the pupil smaller.
  • In dim light radial muscles contract to stretch the pupil outwards making it wider.
  • The circular and radial muscle act antagonistically, when one contracts the other relaxes.


    • The millions of light-sensitive cells in the retina are of two kinds, the rods and the cones.
    • Rods play an important part in night vision.
    • Thought to be three types of cone cells, each responds best to red, green, or blue.
    • If all three types are equally stimulated we get the sensation of white.

  • The central part of the retina is called the fovea.
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