XXXII.] THE EYE. 367 



to be bipolar cells with large nuclei. The processes extend into the 

 inner molecular layer and end in arborisation, and by Golgi's method 

 the opposite processes have been traced outwards as far as the 

 external limiting layer. Other branched cells supposed to resemble 

 neuroglia cells lie amongst these cells. 



(/.) The outer molecular layer is thin, and is composed of the 

 terminal arborisations' of processes from the rods and cones and 

 inner granular layer. 



(g.) The external nuclear layer consists of many more rows of 

 nuclei than in (*). They are the nuclei of the rod and cone fibres. 



(/>.) The external limiting membrane, cut across, and therefore 

 appearing as a thin clean-cut line. 



(/.) The layer of rods and cones. The rods are more numerous 

 than the cones, and the latter are shorter than the former. Each 

 rod and cone consists of an outer and an inner segment. 



(/) A layer of hexagonal pigment-cells, somewhat flattened, 

 which sends pigmented processes or filaments between the rods and 

 cones. The length and condition of these processes depends upon 

 the influences to which the eye has been exposed before death 

 (P- 37)- The pigment granules, some of which are crystalline, 

 exist chiefly in the inner part of the cells, and may extend into the 

 cell processes which pass between the rods and cones. 



(H) Study the successive layers. Although the layer of rods 

 and cones and external nuclear layer have been described as 

 separate layers, in reality they should be studied together. Each 

 rod is continued inwards by a tapering fibre the rod-fibre ; in the 

 course of the latter is an oval nucleus, the fibre itself ending in an 

 arborisation in the outer molecular layer. Each cone is similarly 

 prolonged into a cone-fibre thicker than the rod-fibre which also 

 has an enlargement in it containing a nucleus. The cone-fibre ends 

 like the rod-fibre in the outer molecular layer. These two kinds of 

 nuclei intercalated in the course of the rod and cone fibres make up 

 the external nuclear layer, which as the rods and cones are merely 

 modified epithelial cells must be regarded as epithelial in their 

 origin. In a freshly -teased retina the nuclei of the rod-fibres are 

 marked by alternate transverse light and dim stripes. 



Each rod arid cone consists of two segments. The outer segment 

 of the rods is clear hyaline and transparent, and striated trans- 

 versely, and readily breaks up into transverse discs. During life 

 the external segments of the rods contain the visual purple. The 

 inner segment is wider, granular and striated longitudinally. The 

 cones also consist of a shorter tapering outer segment transversely 

 striated, and an inner bulging larger, longitudinally striated, segment 

 which is continued through the external limiting membrane into 

 the cone-fibre. 



