THE CONNECTIVE-TISSUE FRAMEWORK OF THE RETINA. 277 



The limitans externa is not to be regarded as an isolable 

 membrane. Like the interna, it is composed either of a mem- 

 branous expansion of the radial fibres, or, where such isolable 

 fibres are deficient in the granule layer, of the connective sub- 

 stance investing in manifold-wise the external granules, with 

 their nerve fibres. The connecting substance of the outer gra- 

 nule layer is never absent,* not even at the macula lutea, where 

 it was unobserved besides the long cone fibres, until Merkel 

 demonstrated its existence in the form of delicate sheaths invest- 

 ing these fibres. (See his ' Macula lutea,' p. 7.) 



Where, as in Birds, the radial supporting fibres can be readily 

 observed to pass from the inner into the outer granule layer, 

 each fibre may be observed to branch and form membranous 

 capsules around the external granules and their nerve fibres. 

 If after moderate hardening these granules, and with them the 

 rods and cones, are removed as completely as possible by agita- 

 tion from small fragments of the retina, the supporting tissue 

 alone remains, forming a system of sheaths which only becomes 

 in some measure intelligible on the application of very high 

 magnifying powers. The sheaths themselves exhibit a fine 

 parallel striation, indicating a ftbrillar structure, which does 

 not cease at the membrana limitans externa, which they help 

 to form. For beyond the latter an indefinite number of fine 

 stiff fibrils project (fig. 360, 8), which, grouped into the form of 

 circles, form fibre-crates, cradles or basketworkfr, from which the 

 cones fall out, as has been above described. The appearance pre- 

 sented is just as if these fibrils were continuous with the fibrous 

 sheaths that invest the outer granules. f It was obviously por- 

 tions of these fibrous crates which I formerly depicted^ in 

 specimens prepared from the Fowl as fibres exhibiting a certain 

 connection with the connective-tissue sheaths of the external 

 granule layer, and which were also described by W. Krause 

 under the name of needles,^ who considered them to be a 

 constant element of the bacillar layer. The illustration of M. 



* W. Krause's Widerspruch membrana fenestrata, p. 19. 

 t See the illustration in the Archiv fur Mikroskop. Anatomic, Band v., 

 Plate xxii., fig. 4, from Man. 



J Ibidem, Band ii., Taf. xi., fig. 13. 



Membrana fenestrata, p. 6, Plate i., figs. 5 and 7. 



