816 



THE ORGANS OF SENSE. 



ganglionic layer ; a few are centrifugal and end in branched clubbed extremities in the 

 inner molecular or inner nuclear layers of the retina. 



FIG. 688. PERPENDICULAR SECTIONS OF MAMMALIAN RETINA (Cajal). 



A. Layer of rods and cones ; B, Outer nuclear layer ; C, Outer molecular layer ; D, Inner nuclear layer ; E, 

 Inner molecular layer ; F, Ganglionic layer ; G, Stratum opticum ; r, rods ; c, cones, r.g, rod granules ; 

 c.g, cone granules; r.b, rod bipolars ; c.b, cone bipolars ; c.r, contact of rod bipolars with the 

 spherules of the rod fibres ; c.c, contact of cone bipolars with the branches of the cone fibres ; ar, 

 internal arborisation of cone bipolars ; ar', internal arborisation of rod bipolars ; c.n, centrifugal nerve 

 fibre ; h, horizontal cells ; s.s, stratified spongioblasts ; d.s, diffuse spongioblasts ; s.g, stratified 

 ganglion cell ; M, Sustentacular fibre. 



2. Ganglionic or nerve-Cell layer. The cells of this stratum vary in size, are oval 

 or piriform in shape, and form a single layer, except at the macula lutea, where several 



strata are present. Each cell contains a large nucleus, and 

 gives off, from its inner surface, an axon which is continued as 

 a fibre of the stratum opticum. From the outer surface of 

 each cell numerous dendrites arise, which form arborisations in 

 the inner molecular layer. The cells may be divided into uni- 

 stratified, multi-stratified, and diffuse, according as their den- 

 drites ramify in one or in several strata of the inner molecular 

 layer, or extend throughout neafly its whole thickness. 



3. Inner molecular or inner plexiform layer. This 

 is constituted chiefly by the interlacement of the dendritic 

 arborisations of the cells of the ganglionic layer with those of 

 the inner nuclear layer, and has been divided by Ramon y Cajal 

 into five strata. It sometimes contains horizontal cells (spongio- 

 blasts), whose branched processes ramify in it. 



4. Inner nuclear layer or layer of inner granules. 

 This is the most complicated of the retinal strata, and consists 

 of numerous cells which may be divided into three groups, viz. : 

 (a) bipolar cells, (6) horizontal cells, and (c) spongioblasts, or 

 amacrine cells. 



(a) The bipolar cells, by far the most numerous, are fusi- 

 form and nucleated, and each gives off an external and an 

 internal process. The internal processes terminate in flattened 

 tufts, at different levels, in the inner molecular layer, while the 

 external produce an abundant ramification in the external zone 

 of the outer molecular layer. These bipolar cells are divided 

 into rod bipolars, cone bipolars, and giant bipolars. The rod 

 bipolars end peripherally in vertical arborisations around the 

 button-like ends or spherules of the rod fibres, and, centrally, 

 j n branched extremities which mostly become applied to the 

 cells of the ganglionic layer. The cone bipolars end peripher- 

 ^\\j m flattened arborisations in the outer molecular layer, 

 in contact with the ramifications of the foot-plates of the cone 

 fibres, and, centrally, ramify in some one of the five strata of 

 the inner molecular layer. The giant bipolars form, peripher- 



ally, an extensive horizontally arranged arborisation in the outer molecular layer ; centrally, 

 they ramify in one or other of the strata of the inner molecular layer. 



FIG. 689. 



A, A cone and two rods from the 

 human retina (modified from 



port ot^od'lparSed hrto 



