Ramsey, Optic Lobes of Amblyopsis. 43 
tinue forward and downward to the optic chiasma as the optic 
tracts. The fifth layer is composed of diagonal fibers and de- 
scending fibers. These latter nerves pass downward and be- 
come a part of the optic tract (Fig. 6, 5). 
As has been said, the wall of the optic lobes of Ambly- 
opsis has undergone considerable shrinkage in thickness. The 
outer layer is not changed. The second layer, which is derived 
from the optic nerve, is entirely wanting. The optic nerve is 
represented by a small bundle of tissue, which is probably the 
remnant of the neurilemma. Inthe brain where the second 
layer should be, there is a narrow space containing practically 
no tissue. The third: layer is unchanged. The fourth layer 
consists normally of two sub-layers; the outer one has both 
nerve fibers and nerve cells,—the latter according to KRAusE 
being the terminal stations of the optic nerve—and the inner 
sub-layer has the terminal stations of the fifth layer init. The 
outer sub-layer is entirely atrophied in the lobes of the blind 
fish, and the inner one, if at all: present, is indistinguishable 
from the third layer (Fig. 8, 3 and 4). 
The fifth layer is reduced to diagonal fibers. The descend- 
ing fibers which join the optic tracts are atrophied. The diag- 
onal fibers are more apparent than in the normal brain. These 
fibers form a broad commissure in the torus longitudinalis which 
runs laterally to the outer edge of the lobes where it turns back 
into the substance of the brain just beneath the ventricle and 
becomes diagonal. Cross sections of its fibers arising from vari- 
ous levels of the lobes are shown in (Fig. 8, 5). 
The sixth layer isa granular layer. Its thickness is less 
than in the normal brain. No other change is noticeable. The 
thickness of the seventh layer, ependyma, is not more than half 
that of a normal brain. The cells show some shrinkage. 
The differences in the lobes thus appear to be: first, in the 
atrophy of the second layer ; second, the outer sub-layer of the 
fourth layer is entirely gone; third, the descending fibers of 
the fifth layer are wholly wanting ; fourth, the granular layer is 
not so thick and the ependyma is not only thinner but reduced 
in the number of its cells. 
