On the retina of amphibia and reptiles / by J. W. Hulke.

  • Hulke, J. W.
Date:
[1867]
Catalogue details

Licence: Public Domain Mark

Credit: On the retina of amphibia and reptiles / by J. W. Hulke. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    figs. 2—12 c). Its colour is yellow or pale green in the toad, frog, triton, salamander, blind worm, and lizard; while the land and water tortoises and the turtle have also ruby beaded cones, the green beads lying in the smallest cones, the yellow in the intermediate, and the ruby in the largest. The occurrence of the cone-bead in the blind worm is interesting in connection with its other lacertian affinities. It is wanting in the common snake and viper. Nothing is yet known of the chemistry of the colouring matter, or of the physiological meaning of these remarkable beads. I have found that rose-aniline imparts to the yellow and pale green beads the intense stain it gives the nucleus of the red blood corpuscle, and that iodine turns the ruby to mauve and the yellow to dark green. In the emys the red beads absorb the violet, indigo, blue, green, and part of the yellow rays of the spectrum to the d (soda) line, and transmit the remaining yellow, orange, and red rays. The yellow beads absorb the violet and other rays to about the b line of the spectrum, and transmit part of the green, and all the yellow, orange, and red rays. The green beads absorb the violet, indigo, and a small adjacent portion of the blue rays of the spectrum, and transmit all the rest. In short it appears that the beads absorb the rays belong- ing to the chemical, and transmit those belonging to the heat-domain of the spectrum1. Whatever other differences are held respecting the mode of visual perception, all physiologists have long agreed that the cones and rods are the sentient elements. This is concluded (1) from the absence of perception from the optic nerve-entrance, where there are no cones or rods; (2) from the unfitness of the other nervous tissues, by reason of their anatomical arrangement, to receive separate and distinct stimuli; (3) from the obvious fitness of the cones and rods by their serial arrangement in a single stratum, and their insulation, to receive distinct stimuli; (4) by the formation of a superficies by the outer ends of the shafts collectively, upon which correct images may be formed; and (5) by Purkinje's experiment, which demon- strates that the sentient elements lie at the outer surface of the retina. Universal and decisive characters by which we may distinguish cones from rods are yet wanting, if indeed such will ever be dis- covered. Size, shape, and the cone-bead will not always serve us, for while the rods much exceed the cones in the toad, frog, triton, and salamander, their sizes differ little in the tortoise and turtle, and in the chameleon's central fovea the cones are as long and as slender as the most typical rods. Again, the bead, which is perfectly 1 For the use of several spectroscopes and much valuable assistance in these obser- vations, I am indebted to Mr Heisch.