NERVOUS CONSTITUENTS OF THE RETINA. 



240 



Besides this structure, so characteristic of the outer segments 

 fresh or well-preserved specimens also exhibit a longitudinal 

 striation.* This, as Hensen first recognized, depends upon the 

 presence of a number of folds running in the direction of the 

 long axis, or in elongated spirals which at the same time vary 

 in depth. As it frequently occurs that in rods preserved in 

 osmic acid, friction or pressure causes the separation of disks 

 of varying thickness, which turn their flat surfaces to the 



f 



Fig. 353. Outer segments of rods and cones, a d, Rods from the 

 Frog ; e, from Man ; /, twin-cone from a Fish (Perch) ; a, fresh rod 

 still connected with the internal segment (s', lenticular body) ; 6, first 

 stage of imbibition in serum ; c, the same stage in dilute solution of 

 potash, magnified 500 diameters ; d, disintegration of a rod into 

 disks in serum, magnified 1,000 diameters ; e, appearance presented 

 by a rod from the retina of Man immersed in strong solution of 

 per osmic acid immediately after enucleation, and macerated for 

 twenty-four hours, magnified 1,000 diameters ; /, fresh rod examined 

 in serum. 



observer, it is easy to obtain a clear image of the relief of the 

 surface. In such disks, shown in fig. 354, besides the grooving 

 of the surface, there is some indication of radial cleavage of the 

 substance of the rod proceeding from the bottom of the grooves. 

 Fresh rods examined in serum exhibit here and there longitu- 

 dinal fissures. The appearances presented by the surface of 

 the disks, which call to mind blood corpuscles, the edges of 



* Hensen in Virchow's Archiv, Band xxxix., Taf. xii., fig. 7. Max 

 Schultze, Archivfiir Mikroskop. Anatomic, Band v., Taf. xxii. 



