252 THE KETINA, BY MAX SCHULTZE. 



guished in the rods of Man or of Mammals. The existence of 

 an axial fibre in the inner segment leading up to the ellipsoidal 

 body like the supposed fibre of the outer segment, is however 

 very doubtful. I have myself been unable to discover any 

 axial fibre in the rods of Man. 



The internal segments of the rods and cones in Man and 

 many animals, on the other hand, exhibit, when very completely 

 preserved in perosmic acid, and carefully examined with the 

 highest powers, a fine longitudinal striatlon on the surface, * 

 which recals that above mentioned of the outer segments of 

 Amphibia, and is in fact, partially at least, continued on the 

 latter. f Even if in these last, however, it is impossible to 

 demonstrate that the striae ar& independent structures by iso- 

 lating them in the form of fibres, the impression given being 

 due rather to simple channelling of the surface (see above), it is 

 nevertheless certain that there are points at which fine fibrils 

 producing the appearance of striation are capable of being strip- 

 ped off the internal segments. In the large cones of the human 

 retina, the striation of the surface is under certain circumstances 

 very distinct. The striae run longitudinally or in long spirals, 

 and are from forty to fifty in number ; they are placed at equal 

 distances from one another, and this amounts at the widest 

 part of the cones to about half a micromillimeter. At the 

 apex of the internal segment they become so crowded, that 

 with the optical means at present at our disposal it is impos- 

 sible to distinguish them. 'Yet the impression is given that the 

 striae are continued in the form of a conical tube on the surface 

 of the outer segment ; for a delicate sheath proceeding from or 

 continuous with the striated cortex of the internal segment may 

 be isolated for a variable distance on the outer. 



Like those of the cones, the inner segments of the rods of 

 Man and of Mammals present a striated surface. The striae, 

 which are eight or ten in number, run in the form of extremely 

 fine lines placed at equal distances around the internal segment, 

 and parallel to its long axis, or, as in the cones, in elongated 



* Max Sclmltze, Archiv fur Mikroskop. Anatomic, Band v. , p. 394, 

 Taf. xxii. 



f Hensen made the first observation on this point in the Frog. 

 Virchow's Archiv, Band xxxix., p. 489. 



