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Page:Popular Science Monthly Volume 44.djvu/387

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LOGICAL METHOD IN BIOLOGY.

power than any other set of investigators, but they had certainly not made the most of it. Steinach exhausted the conditions by taking into account, one at a time, the different degrees of inherent excitability, effect of exposure to and exclusion from light, and the reactions of the normal eye, the excised eye, and the isolated iris, against the different degrees of light, thus:

Diffused light.
Normal eye. Concentrated gaslight.
Excluded from light. Concentrated sunlight.
Exercised eye.
Frog's eye. High excitability. Isolated iris.
Exposed continuously
to light.
Medium excitability.
Low excitability. Etc.

These do not include all the conditions which he detected, but they are sufficient to indicate the difference between his method and that of his predecessors. The modifying conditions were not discovered in the order in which they appear in the table, but tabulation shows very quickly whether or not they have been exhausted.

When all the favorable conditions were combined there invariably resulted a characteristic contraction of the pupil, on exposure to light, whether the object experimented on was the normal eye, the excised eye, the isolated iris, or the isolated iris deprived of its ciliary rim. In other words, the contraction of the pupil in the excised eye of fishes and amphibia does not depend on an intraocular reflex involving the retina, but on the direct influence of light on one or more of the elements of the inner or pupillary part of the iris. It had been suggested that the phenomenon was due to the action of light on the endings of the nerve fibers in the sphincter muscle of the iris. Steinach removed this suggestion from the group of remaining possibilities by paralyzing the nerves of one eye of an animal with atropine and leaving the other normal, and showing by comparative tests that the two eyes continue to act alike. He showed by a special experiment that the posterior pigment layer of the iris has nothing to do with its contraction. The branched or stellate pigment-cells—the chromatophores—in the front part of the iris were possible factors in the problem. They were known to undergo changes due to the action of light. Light causes a redistribution of the pigment within the cell, causing it to collect at the center. When the eye of an animal which has been kept in the dark is alternately shaded and exposed to the light, there follow a prompt alternate dilatation and contraction of the pupil. This process can be carried on for some time before there is any visible change in the chromatophores; at the end of half an hour or more the chro-