950 
MR. A. SANDERS ON THE ANATOMY OE THE 
the tuberculum impar. The only peculiarity about it is that the ganglion of origin of 
the posterior root is situated higher than the corresponding part in M. cephalus , being 
placed above, instead of below, the floor of the fourth ventricle. 
The spinal nerves (figs. 13 and 14) take their origin very much in the same way as in 
the case of M. cephalus. The dorsal roots, after running a short distance dorsad along 
the edge of the section, pass downward and inward to enter the apex of the dorsal horn 
of grey matter by two bundles, and there they are soon quite lost to view. The ventral 
roots are derived from two sources—one from the transverse commissure, which separates 
the two ventral longitudinal bundles ; the other comes from the ventral horn of grey 
matter of the same side. These two bundles join at the ventral edge of the cord and 
emerge together as a single root. The fasciculus from the transverse commissure is 
piobably derived from the ventral horn of the opposite side. 
Homologies of the Brain in the Mormyridae. 
We have now to consider what relation the brain of Mormyrus bears to that of the 
ordinary Teleostei. What are the homologies of those highly developed wings which 
Marcusen, in despair of finding their signification, termed the “ peculiar organ ” 
(Eigen thiimliches Organ) ? What the homology of the tuberculum impar? If we examine 
a longitudinal section (fig. 1) we see at once that they are new formations. We have 
only to remove them and to allow the displaced organs to resume their proper position, 
and we have a brain like that of any other Teleostean. There is a slight difference in 
the direction of the cerebellum, but that occurs in other families, such as the Siluridse. 
In order to solve this problem we have examined the characteristics of this “ peculiar 
organ, and we have found, in the first place, that it contains all the elements of the 
cerebellum arranged in a slightly different manner; secondly, we have found that it is 
not the cerebellum, since that organ exists, presenting the ordinary form and arrange¬ 
ment of tissue; thirdly, that its roots are derived from that part of the brain which 
lies immediately in front of that organ; the tecta lobi optici occupy this position in 
ordinary Teleostei, but here they are pressed towards the base of the brain without 
being otherwise deformed. The only organ then occupying this position that remains 
for comparison is the valvula cerebelli. This part is highly developed in the Ballan 
Wrasse ( Labrus maculatus), and if we examine a transverse section through it (fig. 15), 
we find in the centre a double layer of the molecular layer indicating the fold of 
cerebellum by which it is formed; on each side we see that the organ stretches out 
into a wing, which is composed of a folded stratum of the molecular layer, below which 
the remainder of its bulk is made up of the granular layer. Even here it looks as if an 
attempt had been made to pack a large amount of tissue into a small space. 
We now turn to a section (fig. 3) through the brain of a Mormyrus , taken perhaps 
a little farther in advance. We find here the same arrangement; in the centre there 
is a double molecular layer below and a single layer above; this is accounted for by 
the greater number of foldings undergone by the molecular layer, and would explain 
