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retinene alters the permeability of the rod when it is dissociated from 

 the opsin. The rhodopsin in a fixed rod has been shown by electron 

 microscopy to be arranged in a regular array of disks which essentially 

 fill the rod. The arrangement of rhodopsin molecules within the disks 

 is not known. The disks are about 250 nu* diameter and 50 m/n thick. 

 It is possible that this array of disks acts somehow as a semiconductor 

 (or even transistor) whose conduction depends on the number of im- 

 purity centers (dissociated retinene molecules). But this cannot be 

 proved. In common with hearing, olfaction, and taste, it is impossible 

 to describe the method by which the neural impulse is started. 



REFERENCES 



1. Wolken, J. J., ed., " Photoreception " (Monograph) Ann. New York Acad. Sc. 

 74: 161-406 (1958). 



2. Wald, George, "The Biochemistry of Visual Excitation," O. H. Gaebler, 

 Enzymes : Units of Biological Structure and Function (New York : Academic 

 Press, Inc., 1956) pp. 355-367. 



