THE EYE. 835 



The outer process passes outward into the outer molecular layer, and there breaks 

 up into a number of branches. According to Cajal, there are two varieties of these 

 bipolar cells : one in which the outer process arborizes around the knobbed ends of 

 the rod-fibres, and the inner around the cells of the ganglionic layer these he 

 calls rod-bipolars (Fig. 448, 1, c, d) ; the others are those in which the outer 

 process breaks up m a horizontal ramification, in contact with the end of a cone- 

 fibre ; these are the cone-bipolar^ and their inner process breaks up into its terminal 

 ramifications in the inner molecular layer (Fig. 448, 1, ). (2) At the innermost 

 part of this inner nuclear layer is a stratum of cells, which are named by Cajal 

 amacnne cells, from the fact that they have no axis-cylinder process, but they give 

 a number of short protoplasmic processes which extend into the inner molecular 

 layer and there ramify (Fig. 448, 1, h). There are also at the outermost part of 

 this layer some cells, the processes of which extend into and ramify in the outer 

 molecular layer. These are the horizontal cells of Cajal. (3) Some few cells are 

 also found in this layer, connected with the fibres of Miiller, and will be described 

 with those structures. 



6. The outer molecular layer is much thinner than the inner molecular layer; 

 but, like it, consists of a dense network of minute fibrils, derived from the proc- 

 esses of the horizontal cells of the preceding layer and the outer processes of the 

 bipolar cells, which ramify in it, forming arborizations around the ends of the rod- 

 fibres and with the branched foot-plates of the cone-fibres. 



7. The Outer Nuclear Layer. Like the inner nuclear layer, this layer contains 

 several strata of clear oval nuclear bodies ; they are of two kinds, and on account 

 of their being respectively connected Avith the rods and cones of Jacob's membrane 

 are named rod-granules and cone-granules. The rod-granules are much the more 

 numerous, and are placed at different levels throughout the layer. Theil- nuclei 

 present a peculiar cross-striped appearance, and prolonged from either extremity 

 of the granule is a fine process : the outer process is continuous with a single rod 

 of Jacob's membrane; the inner passes inward toward the outer molecular layer 

 and terminates in an enlarged extremity, and is embedded in the tuft into which 

 the outer process of the rod-bipolars break up. In its course it presents numerous 

 varicosities. The cone-granules, fewer in number than the rod-granules, are placed 

 close to the membrana limitans externa, through which they are continuous with 

 the cones of Jacob's membrane. They do not present any cross-striping, but con- 

 tain a pyriform nucleus which almost completely fills the cell. From their inner 

 extremity a thick process passes inward to the outer molecular layer, upon which it 

 rests by a somewhat pyramidal enlargement, from which are given off numerous 

 fine fibrils, which enter the outer molecular layer, where they come in contact with 

 the outer processes of the cone-bipolars. 



8. The Membrana Limitans Externa. This layer, like the membrana limitans 

 interna, is derived from the fibres of Miiller, with which structures it will be 

 described. 



9. Jacob's Membrane (Layer of Rods and Cones). The elements which com- 

 pose this layer are of two kinds, rods and cones, the former being much more 

 numerous than the latter. The rods are of nearly uniform size, and arranged 

 perpendicularly to the surface. Each rod consists of two portions, an outer and 

 inner, which are of about equal length. The segments differ from each other as 

 regards refraction and in their behavior with coloring reagents, the inner portion 

 becoming stained by carmine, iodine, etc., the outer portion remaining unstained 

 with these reagents, but staining yellowish brown with osmic acid. The outer por- 

 tion of each rod is marked by transverse striae, and is made up of a number of thin 

 disks superimposed on one another. It also exhibits faint longitudinal markings. 

 The inner portion of each rod. at its deeper part where it is joined to the outer 

 process of the rod-granule, is indistinctly granular ; its more superficial part pre- 

 sents a longitudinal striation, being composed of fine, bright, highly refracting 

 fibrils. The visual purple or rhodopsin is found only in the outer segments of 

 the rods. 



