- - -s - - ; - r 



3-- 



mented sheath as the only possible structure in which the nerve 

 can terminate. 



The view that the great rods are the nerve terminals ap- 

 parently originated from Miiller's statements. Miiller [198J 

 evidently thought the great rods mere bundles of nerve-fibres ; 

 of end organs in the Arthropod eye he knew nothing, and in his 

 text-book on Physiology [192] he apparently accepted Wagner's 

 [200] theory, that the cones are invested by the fibres of the 

 optic nerve. Leydig [207] even went so far as to believe the 

 crystalline cones to be^nerve-terminals. Evidence in favour of 

 such views is entirely wanting. Max Schultze [210] could not 

 agree that either the great rods or cones are nerve terminals, 

 but he ascribed this function to certain spindle-shaped struc- 

 tures within the great rods in the Lobster. The most recent 

 researches [247] on the structure of the eye of the Lobster 

 appear to me to indicate that these spindles are not part of the 

 great rod, and, although the investigation is beset with technical 

 difficulties, it appears to me probable that it will turn out that 

 the structures described by Max Schultze are really retinal, 

 and correspond with the nerve-end organs which I regard as 

 the retina in Insects. 



If we except the doubtful evidence afforded by Max Schultze, 

 who, as I think, erroneously regarded his spindles as part of 

 the great rods, there is not the slightest evidence, in any single 

 work published before Grenacher's monograph appeared, that 

 the optic nerve terminates in the dioptron, and this renders it 

 the more difficult to understand the ready acceptance of the 

 theory by Grenacher. 



Since the appearance of Grenacher's monograph several 

 investigators have attempted to prove that the nerve fibres end 

 in the dioptron. Patten remarks [239, p. 069] : ' It cannot be 

 said th;u Grenacher or any of his predecessors, perhaps with 

 the exception of Max Schult/e, who has represented the fibrous 

 markings of the style and calyx (the retinula), have succeeded 

 in demonstrating anything like nerve endings in the Arthropod 

 eye.' With regard to Patten's own statements, I do not for a 

 moment deny the existence of such a network of fibres as 



