522 THE SENSES AND SENSORY ORGANS. 
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 [198] 
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. 669]: ‘ It cannot be 
said that Grenacher or any of his predecessors, perhaps with 
the exception of Max Schultze, 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 
