SCHULTZE, ON THE RETINA. 29 
a different manner from that composing the bodies. This substance 
does not become granular, like protoplasm, but either hardens into a 
homogeneous mass or shrinks and curls up in a peculiar manner, at 
the same time cracking, generally transversely, but sometimes also 
longitudinally. That an external tunic and contents—a cortex and 
central filament—can be distinguished in them I hold to be highly 
improbable. The outer segments of the ‘rods’ are cylindrical, though 
a very slight attenuation towards the choroid may occur (frog); on 
the other hand, the outer segments of the ‘cones’ are of a decidedly 
conical form, the apex pointing outwards, and usually terminating 
below the summits of the rods. 
«2. Avery remarkable difference between the ‘rods’ and ‘cones’ is 
presented in the filaments proceeding from them to the external gra- 
nule-layer. The filaments belonging to the ‘cones’ are of considerable 
thickness, which sometimes is as much as 2—5 micro-millimeters ; 
they exhibit here and there a delicate longitudinal striation, as if they 
were composed of parallel fibrils; and they always break up on the 
upper surface of the intergranular layer into an indeterminate num- 
ber of extremely delicate fibrils, which are lost in that layer.* The 
fibres proceeding from the rods, on the contrary, have a scarcely mea- 
surable thickness, and they can only be traced to the surface of the 
intergranular layer, where they apparently terminate in a minute 
enlargement whose nature is at present obscure. Each filament, 
whether belonging to a ‘ cone’ or ‘rod,’ is in some part of its course 
connected with a cell—an outer granule—so that the outer granules 
may be divided into ‘rod’ and ‘cone-granules,’ of which the latter, 
at any rate in the mammalia, are the larger. Both kinds of filaments 
present all the characters of nerve-fibres, and much resemble those of 
the optic nerve-layer, and, on the other hand, they are manifestly dis- 
tinguishable from those of the trabecular framework. 
3. At the yellow spot of the human and simian retina ‘cones’ 
only exist. Close to its periphery, however, ‘rods’ become inter- 
posed between them, and at a few millimeters from the middle of the 
spot they are present in the number of two to three between each 
two ‘ cones,’ a proportion which is continued uninterruptedly up to 
the ora serrata. In proportion as they become crowded together at 
the macula lutea, their fibres, as well as those of the ‘ rods’ interspersed 
among them, assume an oblique direction, radiating, as it were, 
* In a valuable communication to the Royal Society, read in June, 1866, 
on the ‘‘Chameleon’s Retina,” Mr. Hulke states “ that from the inner ends 
of the cones fine fibres proceed obliquely from the outer to the inner sur- 
face of the retina in a radial direction from the centre of the fovea to the 
periphery of the retina.” These fibres connect the cones with the cells of 
the outer granule-layer; they next form a thick plexus at the inner surface 
of this layer, which he terms the ‘‘ cone-fibre-plexus ;” then traverse the inner 
granule-layer, in which they connect themselves with round and roundly oval 
cells, and are continued through the medium of the ganglion-cell-like cells of 
this layer into the granular (molecular layer, Schultze), where they join the 
processes directed outwards from the cells of the ganglionic layer. ‘ Thus,” 
he says, “they constitute an anatomical path between the cones and optic 
nerve-fibres.” 
This, if confirmed by future observation, isa most important fact, and one 
of great import with relation to the apparently more direct and immediate 
communication between the “cones” and optic nerve-fibres than would 
seem to obtain with respect to the “rods.” 
