Re: Why do pupils dilate?

pupil motion is intended as a regulation, to allow a fixed amount of light 
to hit the retina. When you are in a dark ambient, the diameter of the 
pupilla increases, and, symetrically, decreases when the ambient light is 
high. This regulation spans through a fixed range, i.e. you are unable to 
keep your vision in an "absolute" darkness.
pupil's motion is under supervision of a reflex arch, wich takes its origin 
in the retina, the effector being the sphincter (an out of will muscle, 
i.e. you cannot move it on a voluntary basis).
Pupils are always symetric, regardless of the ambient light. Asymetry means 
a disease is present.

Hope this helps.

Luc

Dr Luc Ronchi
Paed Anesthesia
St Nazaire, France