Page:Popular Science Monthly Volume 34.djvu/178

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dermis, set on the extremity of a pineal outgrowth from the brain. The clear area around it is caused by the dermis, which is transparent and free from the pigment which covers it internally in other parts. The eye is covered by an escutcheon-shaped epidermal shield, more transparent in the center and larger (three by three millimetres) than the normal epidermal scales. The only sign of degeneracy is the central cloudy mass of pigment, like a big cataract."

I was naturally desirous of determining for myself how far it was sensitive to light, but found the investigation beset with difficulties. Chloroformed lizards that were deprived of their eyes, although the amputation was dexterously performed, did not revive sufficiently to make their subsequent movements suggestive; or did sympathetic ophthalmia set in and affect the pineal eye?

I subsequently hit upon a plan, using very thin India-rubber cloth, by which the eyes proper were effectually closed, and the "eye" of the vertex left free. The lizards thus provided with a blinding head-gear were separated from their fellows and placed in a roomy inclosure, made up of several almost dark and very light alternate sections, the temperature being even throughout the lizards' range. The arrangement was, perhaps, too artificial for a satisfactory series of observations, but it became evident at once that the lizards recognized the difference between the dark and light areas, and their prompt return to the latter when removed from them, and again their actions when they returned, all showed the appreciation of a difference, which I know was not one of temperature, but beyond this I could determine nothing; but I recalled, at this juncture, the significant fact that in the woods about May's Landing I noticed many lizards buried in the fine sand and leaf-mold, their eyes closed and covered, but the top of the head and a portion of the back, for its whole length, exposed. The same was subsequently noted as a position frequently assumed by the lizards in my Wardian cases. If, therefore, the "pineal eye" is sensitive to light, it is still of some use to the creature, as it would certainly respond to a passing shadow, and so warn the animal of the approach of a possible enemy. It certainly would be greatly to the lizard's advantage if it had a perfect eye in the top of its head, especially when it rests upon the trunks of trees, and is exposed to the attacks of predatory birds; but the "pineal eye" is at most but a remote approach to this. On the other hand, it was found that whenever I converged the rays of light with a burning-glass, always so suddenly that no thermal effect was produced, there was caused a movement of uneasiness, a flinching, on the part of the lizard that was extremely suggestive.