Can the Response of the Iris to Light be Used to Break the Code of the Second Cranial Nerve in Man

The sphincture muscle of the vertebrate iris develops tension causing the aperture stop of the eye to become small in the light, and loses tension making it expand in the dark. In this way, the light available to the sensory retina can alternatively be diminished when the world is too bright and increased when it is too dim. This elegant servomechanism is a textbook example, so we are told, of a homeostatic mechanism maintaining the exquisite sensitivity of the vertebrate retina constant while environmental light fluctuates. Nonsense! The maintenance of good retinal sensitivity over a range of 10,000,000,000,000 fold from the one extreme near the theoretical limit of detection to the other extreme at which the light becomes so intense that the retina starts to burn, is no doubt a prize achievement of Mother Nature as a bioengineer, but the photopupillary response has precious little to do with it. The small fluctuations of pupil area available (about 1.2 log10 units, for example, for the human eye) is trivial in this context.