NERVOUS CONSTITUENTS OF THE RETINA. 251 



the still well preserved extremities of the rods in their natural 

 state, it will be found that, by careful focussing, a black point or 

 short line becomes occasionally visible in the centre of the rods,* 

 and this may be attributed to the presence of an axis fibre. 

 Hensenf firmly maintains, in opposition to Krause,^: that the ap- 

 pearance in question is really caused by a pre-existent structure. 

 A perfectly satisfactory explanation of its occurrence has not yet 

 been given ; for no one has hitherto demonstrated the presence, 

 by isolating it, of an axial fibre in the outer segment. It is to 

 be observed also, that no trace of an axis fibre or of an axial 

 canal can be discovered in the detached disks even of the 

 thick rods of Amphibia, however perfectly they may be pre- 

 served. (See the above figures:) On the other hand, it must 

 be admitted that, according to the observations of Zenker, there 

 is a difference in the refractive index between the investing 

 layer and the central portion of the rods, which in all proba- 

 bility furnishes an explanation of the appearance in question. 

 These indices are estimated by Zenker to be 1*5 for the maxi- 

 mum, and T33 for the minimum. In the rods of the Frog, || when 

 slightly swollen in perosmic acid, Hensen believed he saw 

 indications of the presence of three axial fibres lying in close 

 apposition.^! 



Hensen** also, with a few other observers, bslieves that an 

 axial fibre may be distinguished in the inner segment of the 

 rods. W. Krauseft first depicted this in the cones of Birds, when 

 he made it terminate in an ellipsoidal body, to which he applied 

 the term optic ellipsoid. We shall return to the consideration 

 of these bodies when speaking of the rods and cones of Birds 

 and other animals ; nothing whatever of them can be distin- 



* Max Schultze, Archiv fiir Mikroskop. Anatomic, Band ii., p. 219, 

 Taf. xiv., fig. -5. Hensen, in Virchow's Archiv, Band xxxix.,p. 486, Taf. 

 xii., fig. 4 A. 



t Archiv fiir Mikroskop. Anatomie, Band iv., p. 347. 



J Membrana Fenestrata, p. 23. 



Archiv fiir Mikroskop. Anatomic, Band iii., p. 259. 



|| Krause more recently estimates the refractive index of the rods to 

 be from 1*45 to 1'47. Membrana fenestrata, p. 25. 



IT Virchow's Archiv, Band xxxix., p. 489, Taf. xii., fig. 8. 



** Loc. cit, fig. 6. 



+f Anatom. Untcrsuch., 1860, Taf. ii., figs. 5 and 6. 



