Pupils

The iris acts like the aperture of a camera: the pupil width adapts to the always different lighting conditions. The parasympathetic nervous system causes reflex contraction of the ring musculature when the brightness increases in the context of close adjustment and in cases of severe fatigue. This narrows the pupil and reduces the incidence of light. This process is called Miosis. In reverse stimuli, the radial muscle fibres contract under the influence of the sympathetic nervous system and the pupil dilates. This process is called mydriasis and some medications also have an influence on the pupil width. For example, atropine dilates the pupils, while opiates constrict them.