NERVOUS CONSTITUENTS OF THE RETINA. 241 



case by W. Krause,* constitute a counterproof against the 

 nervous nature of the cone fibres. 



The rod fibres, again, can only be followed as far as, or just 

 into, the external granulated layer. Our knowledge of their 

 mode of termination is, however, still more imperfect than in 

 regard to the cone fibres. The great delicacy and destructi- 

 bility of the internal portion of the rod fibres in Mammals and 

 Man only permit them to be exceptionally preserved through- 

 out their whole course to the external granulated layer. Very 

 frequently, and especially where numerous and large varicosi- 



Fig. 350. 



Fig. 350. Cone fibres, with and without varicosities, from the 

 inner part of the external granule layer of the macula lutea of Man. 

 Magnified 1,000 diameters. 



ties appear in the course of the rod fibre, it terminates just 

 above the external granulated layer with a clavate enlarge- 

 ment.f This gives the impression of being a diminutive re- 

 presentative of the cone-fibre enlargement, though I have not 

 been able to observe any division of the rod fibres into the fine 

 fibrils so characteristic of the cone-fibre spherule. In Fishes, 

 however,:}: and still more in Birds and Amphibia, there is no 

 doubt upon the point. The external granule layer in the last- 

 named animal's consists, for the most part, of only two layers 



* Membrana, fenestrata. 



t Max Schultze, Archivfiir Mikroskop. Anatomie, Band ii., Taf. x., fig. 

 1 ; Hasse, loc. cit., p. 248. 



t Max Schultze, Archiv fur Mikroskop. Anatomic, Band ii., Taf. xi., 

 figs. 8 and 9. 



Idem, figs. 18 and 19. 



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