952 
MR. A. SANDERS ON THE ANATOMY OP THE 
show that with a slight alteration all these layers may be made to do so. In the 
Carp we find that the fourth layer is composed of a granular material in which large 
cells are dispersed ; we have only to imagine these cells to be collected at the summit 
instead of at the sides, when they would homologise with the club-shaped dorsal end 
of the outer layer and the anterior side of the tuberculum impar of the Mormyrus. 
After this our course is plain, and making allowance for its enormous development, we 
have layers in tolerably exact correspondence with each other. The outer layer in 
the vagal tuberosities of Cyprinus is evidently the homologue of the outer layer of the 
Mormyrus, then comes the finely granular layer; the third and fourth layers in Cyprinus 
being by some means displaced, we immediately come upon the outer granular layer of 
the tubercle, which in the places where it is free, extends round the whole circumference : 
this would correspond to the fourth layer of the tuberculum impar. It may be supposed 
that the inferior fibres have become developed at the expense of the upper ones, and 
so would come to be derived from the inner side of this granular layer, and thus corres¬ 
pond to the fifth layer in Mormyrus. We have now only the finely granular interior of 
the tubercle remaining to be accounted for : this would answer to the sixth layer, 
which is the finely granular material forming the central portion of the anterior 
part of the tuberculum impar, and which presents the same relation to the remains of 
the fourth ventricle, and to the central longitudinal columns of the medulla oblongata 
in both Fishes. From these considerations, the conclusion is obvious, that the tuber¬ 
culum impar of the Mormyridce corresponds to the tuberculum impar situated in the 
fourth ventricle of Cyprinus together with the tuberosities of the vagus; the three 
bodies which are nearly separate in Cyprinus having become indissolubly united in 
Mormyrus. 
I have thus, as far as I am able, explained the special formations in the brain of the 
Mormyrus ; but while it is comparatively easy to account morphologically for these 
peculiarities, it becomes very much more difficult, if not impossible, with our present 
amount of knowledge, to account for them physiologically. What can possibly be the 
function of this enormously developed organ and of tb e innumerable repetitions of the 
same structure ? As far as I could learn there was nothing extraordinary about the 
habits of these Fishes that could throw any light on it. It is trne that their eyes are 
very small, while their organs of hearing are comparatively highly developed, as was 
shown by Fischer,' 55 ' but there does not seem to be any connexion between these two 
series of facts. Other Fishes have complicated organs of hearing, and, as far as I know, 
neither these nor the blind or semi-blind Fishes inhabiting caves have any such pecu¬ 
liarity in the structure of their brain. But the most curious fact about these Fishes is, 
that notwithstanding the enormous development of the brain, the nerves emerging 
therefrom have nothing like a corresponding amount of development; most of them 
are quite small, some escape detection, and the only nerves of any size, the trifacial 
and the vagus, do not present anything out of the common, either in extent or dis- 
* Ueber d. Gehororgan der Fischgattung Mormyrus. Freiburg, 1854. 
