THE DUPLICITY THEORY 211 



these cases and regards them as seriously opposed to the duplicity 

 theory. Taken alone the phenomena associated with night blindness 

 are by no means unadulterated evidence in favour of the theory, but 

 viewed in conjunction with the other facts already discussed they are 

 on the whole confirmatory and highly suggestive. Moreover it is by 

 no means certain that in diseases accompanied by night blindness the 

 cones remain unaffected. Except in congenital night blindness it is 

 almost certain that they do not. 



The pathology of retinitis pigmentosa^ shows that the condition 

 is primarily due to vascular degeneration in the choroid, whereby the 

 chorio-capillaris is destroyed. The pigment epithelium and the outer 

 layers of the retina are dependent for their nutrition upon the integrity 

 of the chorio-capillaris. From its proximity the pigment epithelium 

 must suffer first. The production of visual purple in the rods is un- 

 doubtedly bound up in the integrity of the pigmented epithelial cells, 

 so that primary disorder of the functions of the rods might be confidently 

 anticipated on pathological-anatomical grounds. It is the rule in most 

 nutritional disorders for the most highly differentiated and complex 

 structures to suffer first. That there is good evidence of a survival of 

 the cone-functions after destruction of rod-functions is evidence of an 

 intermediary process such as that described. 



It must be admitted that the researches of Hess on the comparative 

 physiology of vision show that little support to the duplicity theory can 

 be derived from lower animals. He found {v. p. 132) that all classes 

 of vertebrates possess good powers of dark adaptation, including even 

 tortoises, which possess neither rods nor visual purple. 



The arguments against the duplicity theory have been collected by 

 Siven^. He lays stress upon the peculiarity of the achromatic scotopic 

 " white " or grey, which he calls blue or violet. He thinks that the 

 rods are chiefly concerned in the perception of lights of short wave- 

 length. He finds the strongest support of his view in the colour-fields, 

 which are admittedly difficult to reconcile with the duplicity theory. 

 He admits the absence of Purkinje's phenomenon at the fovea, and says 

 that when colour is no longer perceived centrally it can still be elicited 

 from the periphery, presumably however only by lights of short wave- 

 length. His experiments in conjunction with Wendt on the effects of 

 santonin poisoning and the yellow vision of jaundice afford him additional 



1 Parsons, Pathology of the Eye, n. 602, 1905. 



2 Siven and Wcndt, Skand. Arch. f. Physiol, xiv. 196, 1903 ; Sivcn, op. cit. xvn. 306, 

 1905 ; XIX. 1907 ; Zisch.f. Sinnesphysiol. xlh. 224, 1907 ; Arch, of Ophth. xlh. 2, 1913. 



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