NERVOUS CONSTITUENTS OF THE RETINA. 239 



lated layer. If the whole space between the limitans externa 

 and the external granulated layer is to be named the " external 

 granule layer," it is still to be noted that an internal division 

 of the external granule layer exists here which is free from 

 granules, and has been named by Henle the external fibrous 

 layer. It must be observed, however, that rod and cone fibres 

 occur everywhere in the external granule layer, and therefore 

 in that part also where the special name of external fibrous 

 layer is not applied to it. 



As the adjoining figure shows, the cone fibres are thicker 



Fig. 349. 



Fig. 349. Cone 'and cone fibre, the latter presenting varicosities, 

 from the vicinity of the yellow spot of the retina of Man. Magnified 

 500 diameters. 



than the rod fibres. Both are pale, with a smooth surface, and 

 especially in the case of the thin rod fibres are very easily 

 broken down. Their disappearance in dilute solution of chromic 

 acid or of perosmic acid stands in direct relation with the above- 

 mentioned occurrence of varicosities, since the more dilute the 

 solution the larger do these become, and the fibres ultimately 

 swelling in every part are destroyed. 



These appearances agree completely with those we have 

 observed in the nerve fibres of the retina. The thicker and 

 somewhat more resistant cone fibres undergo similar metamor- 

 phoses to the rod fibres, the distinctness with which this may 

 be seen being proportionate to their length ; it is therefore most 

 obvious at the macula lutea. When moderately hardened, they 

 appear as pale perfectly smooth fibres which never branch, or 

 anastomose, or pass into spongy plexuses, and are thus sharply 

 differ entiated from the radial supporting fibres and the connect- 

 ing substance that surrounds them. In fluids in which the 

 optic fibres exhibit strongly marked varicosities, distinct vari- 

 cosities also usually occur in the cone fibres, leading ultimately 

 to their complete swelling and solution (fig. 349). Lastly, 

 they exactly resemble the thicker fibres of the optic layer, 

 in the circumstance that, under high powers, they present 



