156 
ME. J. LOCKHART CLARICE ON THE STRUCTURE OE 
of the lobe, to the under side of the eye ; while those which proceed from the under 
surface of the lobe, ascend in a similar way between the others to reach the upper side 
of the eye at i". Moreover, at the upper surface, this layer of fibres is curiously folded 
over each end of the lobe, as shown at g g. 
After removing one of the optic lobes with its peduncle and layer of nerves, if a 
thin longitudinal section be made through the middle line, and magnified about seven 
diameters, it presents the appearance delineated in fig. 3. Here it will be seen that the 
optic nerves at j proceed outward in almost straight lines, while those nearer the end 
and at the opposite side of the lobe (1c, 1c, 1c) are successively more curved in then- 
course to join the others for the formation of the fan-like expansion which is attached 
to the globe of the eye. The substance of ; the lobe consists of two distinct portions, 
which differ from each other considerably both in structure and general appearance. 
The outer portion (l, fig. 3) resembles a thin rind or shell, and is very delicate and very 
easily separated from the substance which it encloses. It may be said to consist of 
three concentric layers, — an external layer ( m ), an internal layer (n), and a broad pale layer 
between them, containing two thin and concentric layers (o) of a somewhat darker hue. 
When this outer or cortical portion of the lobe is subjected to higher magnifying 
powers, it is found to possess a very beautiful and a very elaborate structure. Fig. 4, p 
represents a thin vertical section magnified 220 diameters. The first or outer layer (q) 
to which the optic nerves (r r) are attached, is composed of small round nuclei of nearly 
uniform size, together with a few nucleated cells of either an oval or a poly-angular shape. 
Near the surface the nuclei are comparatively few, but increase in number as they de- 
scend, and on approaching the border, which is very sharply defined, they are crowded 
closely together. Through this layer the nerves enter in separate bundles which 
diverge as they descend. The majority, at least, of their fibres are connected with the 
nuclei, and form with them a close network ; but whether any of them run directly 
through to the next subjacent layer I have not determined satisfactorily. 
The second layer (s) is composed entirely of fine nerve-fibres which run in two different 
directions at right angles to each other, the one being vertical, and the other horizontal 
or concentric with the layer. The vertical fibres are by far the more numerous. They 
issue from the under surface of the first layer as prolongations of the nerve-roots, but 
they have no fascicular arrangement, being uniformly disposed in parallel lines. Some 
of them, particularly at the lower part, abandon their original direction to become 
continuous with the horizontal fibres ( t , t'), while the rest are prolonged across these 
fibres as far as the next layer (u). The horizontal fibres, for the most part, are col- 
lected into separate bands. The highest of these ( t ) is very slender, and below it are 
delicate fibres running in the same direction, but with wider intervals between them. 
The other two bands are at the lower end of the layer. They are much broader, and are 
separated only by a narrow interval. The lowest is in contact with the third or sub- 
jacent layer (u). 
This third or internal layer (u, fig. 4, and n, fig. 3) of the cortical portion of the lobe 
