THE OPTIC LOBES OF THE CUTTLE-FISH. 
157 
is composed of round and closely aggregated nuclei of nearly the same size as those of 
the first layer. The nuclei are united in a network of fibres, which at the upper border 
(w) are directly continuous with both the transverse and longitudinal fibres of the layer 
next above. At the lower border ( w ') is a single row of nucleated cells, which send their 
processes upwards, inwards, and sideways. 
But the cortical substance, consisting of these three layers, forms only a very small 
portion of the optic lobe, the chief bulk of which has a structure and appearance of 
quite another kind. From the nuclear network and nucleated cells of the third or 
inner layer ( u , fig. 4) of the cortical portion of the lobe, a continuous series of fine 
nerve-fibres may be seen to issue at its lower border ( w ). At first these fibres are 
vertical, parallel, and arranged side by side in a nearly uniform series ; and between 
them are scattered without regularity a number of round, oval and triangular nuclei like 
those of the cortical layer ; but immediately after their exit the fibres begin to arrange 
themselves in bundles, which, as they descend, deviate from their former vertical direc- 
tion, and decussate each other in a plexus or network (x, x, x) ; while the nuclei, in 
corresponding proportion, collect at first into small irregular groups (y y ) between the 
bundles, and then into groups that are larger and more compact, within the meshes of 
the plexus (y 1 y') ; they are not, however, completely isolated, but communicate with each 
other, to a greater or less extent, by means of lateral offsets. At first the meshes and 
the groups which they contain are more or less fusiform and disposed with their longer 
axes vertical, so as to constitute a system of communicating rays at right angles to the 
cortical layers (y’ y', figs. 4, 3, 5, & 6); but as they reach the centre of the lobe, where 
the bundles of the plexus are more divergent and decussate each other, in every direc- 
tion, at greater angles, the groups contained in the meshes become more globular, more 
isolated, and at the same time larger ; but still they communicate by lateral offsets like 
a number of stellate cells (figs. 3, 5, & 6). Fig. 5 represents a transverse section of the 
lobe along the line a', fig. 2, and shows the globular and stellate form of the cell-groups 
in the centre, with their elongated form and radiate arrangement near the surface*. 
Fig. 6 represents another transverse section through the peduncle and its tubercle, along 
the line V , fig. 2. Here the cell-groups in the centre are still more globular, and con- 
sequently, in section, many of them appear much larger than the elongated groups which 
are cut transversely in fig. 5. 
Under a magnifying power of 60 diameters, thin sections made in different planes 
through the central parts of the lobe present the appearances delineated in fig. 7. Here 
the blank, communicating spaces, so well seen in fig. 5 between the' central groups of 
cells, are occupied by the most intricate part of the plexus, where the bundles interlace 
in every possible direction. The fibres of each bundle are connected with the cells of 
different groups by means of their processes, which are turned to every side. 
On examining the cell-groups under still higher powers, they were found to consist of 
round, oval, pyriform, and triangular nuclei, confusedly mingled with small and large 
* Fig. 5 should have been reversed from right to left by the engraver. 
Y 
MDCCCLXVII. 
