THE EYE. 419 



but the bodies differ in shape, being- bi-convex or rather oval in 

 form. The inner seg-ments are enclosed in a delicate sheath con- 

 tinuous with that of the outer segments. 



In some cases two cones are united to form a twin-cone ; in such 

 cases the one is always larger than the other (Fig. 259 9), and has 

 several peculiarities which distinguish it from the smaller. 



The smaller or secondary member of a twin-cone is longer, and 

 possesses a lens-shaped body which is plano-convex. The lai'ger or 

 principal member of a twin-cone is shorter, has a plano-convex 

 body, but also an oval, homog-eneous, glistening body, which is 

 directly attached to the plano-convex body. The shape of the two 

 members is also different. 



The outer nuclear layer (Fig. 258 _/) is 14-16 [x thick; the 

 nuclei lie in two layers. The nuclei belonging to rods and cones 

 have the same characters, each nucleus being a large, oval, hyaline 

 body, and enclosing a bright nucleolus. Each nucleus is surrounded 

 by an extremely thin layer of finely granular matter. The inner 

 processes of the nuclear bodies both of the rods and the cones extend 

 to the outer molecular layer, are there dilated and serrated, where 

 they become attached to the outer molecular layer (Schultze, Hoff- 

 mann). In some cases, however, the inner process of the nuclear 

 bodies, belonging to the rods, forms only a short fine fibre. 



In the case of twin-cones the corresponding parts in the outer 

 nuclear layer possess two nuclei (Schultze). 



(8) The pigment layer (Figs. 258 //, and 259 n, 12, 13, 14, 15) 

 is not intimately attached to the rest of the retina. It consists of 

 cylindrical cells in which two parts or segments are sharply diffe- 

 rentiated ; the external part, directed towards the choroid coat, is 

 of pale, or colourless granular protoplasm, and occupies one-third 

 of the length of the cell ; this part encloses a large, round, nucleo- 

 lated nucleus. This colourless segment of the cell also includes 

 one or two bright yellow, fat globules (Morano). Seen from the 

 surface the cells are hexagonal (Fig. 259 n). The remaining 

 two-thirds of the cells consists of a brush formed of numerous fine 

 pigmented processes ; the ultimate terminations of the processes, 

 which lie parallel to each other, are frequently unpigmented ; each 

 cell possesses thirty to forty such processes (Morano). 



The processes extend between the rods and cones as far as the 

 external limiting membrane (Figs. 258, 259), or sometimes a little 

 further (Merkel, Morano, Hoffmann). The processes from one 

 pigment-cell surround a number of rods and cones; according to 



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