THE RETINA. 691 
molecular layer, it may be in one stratum (stratified spongioblasts) or in several strata 
(diffuse spongioblasts). 
5. Outer molecular or outer plexiform layer.—This is constituted by the 
Fic. 500.— 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 ; 7, rods ; ¢, cones, 7.g, rod granules : 
Gg, cone granules ; 7.6, rod bipolars ; ¢.b, cone bipolars ; ¢.7, contact of rod bipolars with the 
spherules of the rod fibres ; c.c, contact of cone bipolars with the branches of the cone fibres ; a7, 
internal arborisation of cone bipolars ; a7’, internal arborisation of rod bipolars ; ¢.n, centrifugal nerve 
fibre; /, horizontal cells; s.s, stratified spongioblasts; d.s, diffuse spongioblasts ; s.g, stratified 
ganglion cell ; M, Sustentacular fibre of Miiller. 
interlacement of the dendrites of the bipolar and horizontal cells, just described, with the 
spherules of the rod fibres and the ramifications of the foot plates of the cone fibres. It is 
divided into two strata: (a) external, indicating the contact 
of the rod bipolars with the spherules of the rod fibres ; (4) 
internal, the line of contact between the cone bipolars and 
the branches of the cone fibres. 
6. Outer nuclear layer or layer of outer granules. 
—This is made up of clear granules somewhat resembling 
those of the inner nuclear layer, but divisible into two kinds ; ( 
(a) cone granules, (6) rod eranules. The cone granules are 
the larger, and each contains an oval nucleus; they lie im- 
mediately inside the outer limiting membrane, through which 
they are continuous with the cones of the next layer. Each iN 
is prolonged internally as a straight fibre, which, on reaching 
the outer molecular layer, expands to form a foot-plate, from 
which several horizontal fibrils are given off. The rod 
granules are far more numerous than the cone granules, and 
each contains a small oval nucleus, which is transversely 
striated. Their outer processes are continuous, through the 
outer limiting membrane, with the rods of the next layer; 
while their inner processes pass into the outer molecular 
layer and end in free, unbranched spherules amongst the 
arborisations of the rod bipolars. 
7. Layer of rods and cones.—This consists of two sets 
of structures, viz. rods and cones. Except at the macula lutea oll 
the rods are far more numerous than the cones and assume fa -_— é 
= Ti etevite 
the form of elongated cylinders, while the cones are shorter 
than the rods and taper externally to fine points. Each rod f | 
and cone consists of two segments—inner and outer. The A 
inner segment of the rod only slightly exceeds in diameter its Fic. 501. 
outer segment, whereas the inner segment of the cone greatly A, A cone and two rods from the 
exceeds its outer part. The inner segments of both rods and human retina (modified from 
cones have an affinity for staining reagents and consist of a oe ees Lah 
basal homogeneous portion and an outer longitudinally-striated Pek San 
part, the proportion of the latter to the former being greater 
in the cones than in the rods. The outer segments have not the same affinity for reagents, 
but tend to break transversely into numerous dises (Fig. 501, B). The colouring matter, 
