No. I.] EVES OF ARTHROPODS. I4I 
probably continued into the medulla without losing their iden- 
tity. There is reason to believe, from the similarity in structure 
of the retina and the medulla, that the fibrille of the latter are 
rearranged in a manner corresponding with that of the fibrille in 
the retinal cells. Now if my supposition is correct, we ought to 
find in the medullz of eye I., for example, systems of fibrillae 
similar in number and arrangement to the retinidia of the retina. 
We find indications of such a condition, for there is a furrow in 
the medulla containing a double row of fibres, between which is 
a layer of very fine medullary substance. To make the compari- 
son complete, the cross fibrillae arising from the fibres ought to 
unite with distinct clusters of fibrillz corresponding in compo- 
sition and number with the retinidia on either side of the retinal 
furrow. But although we cannot distinguish these medullary 
vetinidia, since all are united to form one continuous mass, there 
is reason to suppose they really exist, for, as we have shown 
above, where there is a break or any marked peculiarity in the 
arrangement of the retinal retinidia, we find a corresponding 
change in the structure of the medullz. For example, if there 
are more rods and consequently more retinidia on one side of 
the retina, there is an increase in the thickness of the medulla 
on its corresponding side. If there is a bend in the retina, 
there is a similar one in the medulla. If there is a furrow in 
the retina, containing a double row of coarse fibres, there is a 
space in the medulla that also contains a double row of coarse 
fibres, and if the furrow is absent, the space is absent also; 
and we might enumerate a number of other peculiarities in 
the structure of the retina that had their counterpart in the 
medulla. 
I am not prepared to discuss the conflicting hypotheses 
that have been advanced concerning the significance of the 
medullary substauce, but I desire to call attention to the fact 
that its peculiar structure in Acz/zws may throw some light upon 
its function. 
I venture to suggest the idea that occurred to me. The 
medullze may be regarded as the retinas of the mind, the inner 
eyes, in which are reacted the nerve changes produced in the 
external ones. The whole apparatus may be compared to a 
telephone in which a vibration of the air produces a movement 
of the receiver, an intermittent flow of electricity follows, and 
