Figure 1-4. Accommodation for objects 20 feet or more away.
Figure 1-5. Accommodation for objects nearer than 20 feet.
(4) Constriction of the pupil. The circular muscle fibers of the iris also
assume a function in the formation of clear retinal images. Part of the accommodation
mechanism consists of the contraction of the dilator and sphincter muscles of the iris to
constrict the pupil. Constriction of the pupil means narrowing the diameter of the hole
through which light enters the eye. This action occurs simultaneously with
accommodation of the lens and prevents light rays from entering the eye through the
periphery of the lens. Light rays entering at the periphery would not be brought to focus
on the retina and would result in blurred vision. The pupil, as noted earlier, also
constricts in bright light to protect the retina from sudden or intense stimulation.
MD0582
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