THE INTERNAL COAT THE NEEVOUS TUNIC 



651 



level the rod filament ends in a knob-like expansion. Similar knobs may 

 appear along its course. At some point in its course through the-Treclear 

 layer the rod fiber presents a nucleated enlargement, which, under some 

 conditions, shows one to three alternate light and dark transverse stria- 

 tions, the optical expression of the distribution of the chromatin in the 



FIG. 552. FROM THE HUMAN RETINA. 



1, outer nuclear layer; #, outer reticular layer; 3, inner nuclear layer; 4, inner 

 reticular layer; a, external limiting membrane; b, rod cell nuclei; c, cone cell nuclei; 

 d, cone bipolars; e, rod bipolars; /, an exceptionally long process of a rod bipolar. 

 Methylene blue. Highly magnified. (After Dogiel.) 



form of bands. The nuclei of the rod fibers are placed at various levels 

 in the nuclear layer, and collectively occupy nearly its entire thickness. 

 Its outer border, however, contains relatively few rod nuclei. 



THE CONES. The cones resemble the rods in structure, but their 

 cytoplasmic portion is shorter, the inner segment of which is several 

 times as broad (35/u,by7/x). The outer anisotropic segment is espe- 

 cially short and does not contain visual purple, while the isotropic basal 

 segment, whose fibrillated ellipsoid occupies a somewhat greater propor- 



