690 THE ORGANS OF SENSE. 
Our conception of the minute structure of the retina, as of all nervous tissues, has of 
late years been greatly advanced by the discoveries of Ramon y Cajal, on whose observa- 
tions the following description is based (Fig. 500) :— 
The layers from within outwards, ¢.e. from vitreous body to chorioid, are : 
Layer of nerve fibres (stratum opticum),. 
Layer of nerve cells (ganglionic layer). 
Inner molecular or inner plexiform layer. 
Inner nuclear layer or layer of inner granules. 
Outer molecular or outer plexiform layer. 
6. Outer nuclear layer or layer of outer granules. 
7. Layer of rods and cones (bacillary layer). 
8. Layer of pigmented epithelium (stratum pigmenti). 
1. Layer of nerve fibres or stratum opticum.—The fibres of this stratum are 
nearly all centripetal and are mostly continuations of the non-medullated axons of the 
Wy aes cells in the ganglionic layer. Some, however, are 
RY pigmenti centrifugal and end in branched clubbed extremities 
OU Oo bo 
\ _ in the inner molecular or inner nuclear layers. 
(Laxer of 2. Ganglionic or nerve-cell layer.—The cells 
cones of this stratum vary in size, are oval or pyriform in 
| Bots shape, and form a single layer, except at the macula 
limitans lutea, where several strata are present. Each cell 
externa contains a large nucleus, and gives off, from its 
Outer inner aspect, an axon, which is continued as a fibre 
nuclear of the nerve fibre layer. From the outer surface of 
each cell numerous dendrites arise, which form 
arborisations in the inner molecular layer. The cells 
may be divided into unistratified, multistratified, and 
Outer. diffuse, according as their dendrites ramify in one 
layer or in several strata of the inner molecular layer or 
extend throughout nearly its whole thickness. 
To 3. Inner molecular or inner plexiform layer. 
layer -This is chiefly constituted by the interlacement of 
the dendritic arborisations of the cells of the gan- 
Inner iar glionie layer with those of the inner nuclear layer, 
layer and has been divided by Ramon y Cajal into five 
Gana, Strata. It sometimes contains horizontal cells or 
sh spongioblasts, whose branched processes ramify in it. 
4. Inner nuclear layer or layer of inner 
é \ stratum granules.—This is the most complicated of the 
——— = = jpryerm retinal strata and consists of numerous cells, which 
Membrana limitans interna may be divided into three groups, viz.: (a) bipolar 
Ave. COOMA DONE Gaia GEEtnOM laeunnte cells, (6) horizontal cells, and (¢) spongioblasts. 
Human Retina (modified from Schultze). (a) The bipolar cells are fusiform 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, in branched extremities, 
which mostly become applied to the cells of the ganglionic layer. The cone bipolars 
end peripherally in flattened arborisations in the outer molecular layer in contact with 
the ramifications of the cone fibres, and, centrally, ramify in some one of the five strata of 
the inner molecular layer. The giant bipolars form, peripherally, an extensive hori- 
zontally-arranged arborisation in the outer molecular layer; centrally, they ramify in 
one or other of the strata of the inner molecular layer. 
(>) The horizontal cells are of two varieties : (1) small, flattened, star-like cells, lying 
immediately below the outer molecular layer and sending a tuft of dendrites outwards 
towards the bases of the cone fibres, while their axons are directed horizontally for a variable 
distance ; (2) large, irregular cells, lying internal to the above and ending in finger-like 
ramifications in the outer molecular layer. Their axons run horizontally for some 
distance and end in extensive varicose arborisations under the spherules of the rod fibres. 
(c) The spongioblasts are situated in the innermost part of the immer granular layer 
and have not yet been shown to possess axons. Their dendrites ramify in the inner 
