268 THE RETINA, BY MAX SCHULTZE. 



in them or at them. The lamination is here often recognizable with 

 low power, as in the Crabs, since the finest disks do not here exceed 

 0-5 of a micromillimeter, and are united into groups that possess a 

 different aspect, and in the River Crab can even be distinguished by 

 their colour. The exact relation of the nerve fibrils to the lamellated 

 rods is however less known in this case than in the Mollusks. 



Finally, amongst the Yermes, at least in the large-eyed Alciope, the 

 structure of the bacillar layer presents some analogy with that of the 

 higher animals. The rods first observed by Krohn, so far as can be 

 observed in my Neapolitan preparations preserved in various fluids, 

 appear in the form of highly refractile, finely transversely striated, and 

 easily transversely fracturing palisades, which are partly tubuliform, 

 and anteriorly surrounded by pigment. In what manner the nerve 

 fibres of the optic layer situated externally to the palisades terminate 

 in this pigmented bacillar layer, is reserved for future research to 

 determine. 



It may here, however, be mentioned that recently much doubt has 

 been expressed as to the rods and cones really constituting the 

 terminal organs of the optic-nerve fibres. The rod and cone fibres 

 may be of the nature of connective tissue, and may stand in connec- 

 tion with the connective-tissue cells and fibres of the internal layers 

 of the retina. This opinion is maintained by W. Krause, * with 

 whom Landoltf in a certain sense agrees, so far as regards the 

 Amphibia. In Frogs, Tritons, and Salamanders, the external granule 

 layer, as already stated, is so thin, and contains, besides the fusiform 

 rod and cone granules, only such short fibres proceeding from these, 

 that it does not appear to be by any means well adapted to settle the 

 point in question. Landolt moreover admits that these fibres may 

 contain nerve fibres in their interior. This is true also in the case of 

 Birds and Reptiles. In Mammals and in Man, to which Krause's state- 

 ments refer, the difference between the fibres of the connecting sub- 

 stanceand those of the nerve fibres is, in accordance with the description 

 given above, so great, whilst on the other hand the agreement of the 

 rod and cone fibres with nerve fibres is so complete, that it is impossible 

 on anatomical grounds to doubt the nervous nature of the rods and 

 cones. Further researches are required to furnish an explanation of 

 the reason why, after section of the optic nerve, as was first shown by 

 Krause in animals, and in various cases of atrophy of the optic nerve 

 and of the ganglion cells in Man, these bodies do not undergo degene- 



* Membrana fenestrata, p. 48. 



t Arckivfur Mikroskop. Anatomie, Band vii., p. 84. 



