MAUTHNER’S CELL 105 
In addition to the long dendrites, there are small processes 
arising ventrally from Mauthner’s cell and arborizing through 
and about the fasciculus longitudinalis lateralis. In one adult 
brain there were five of these but in most cases I have not found 
more than three. This is in marked contrast to the wealth of 
dendrites in the corresponding region of the cell in Salmo, as may 
be seen by comparing figures 3 and 11 with figure 6. 
The minute dendrites which arise from the region of the axone 
hillock are described in the following section. 
The axone cap. The structure which I am calling the axone 
cap has long been known, but its nature has not been analyzed 
hitherto. In this case, as in several others, Mayser (’82) came 
nearer the truth than anyone else since his day. On the basis 
of preparations of carp brains stained with carmine and with 
osmic acid, he concluded that this ‘Klumpen’ consisted of proc- 
esses of nerve fibers and of ependymal fibers. Beccari (’07), 
working mainly with Cajal preparations, suggested that it might 
be a highly characteristic beginning of the myelin sheath of the 
axone, which as is well known, is very large. This is not the 
case, as may be seen in figure 11, where the sheath (Md.Sh.) of 
Mauthner’s fiber is shown tapering down gradually toward the 
cap,and so far as my observations go, terminating before reaching 
it. Beccari did see some fibers which seemed to come from the 
neighborhood of the fasciculus longitudinalis medialis and appar- 
ently entered the ‘massa,’ or ‘cap’ as I suggest calling it. 
In toluidin blue and erythrosin preparations the axone cap is 
most conspicuous, staining intensely red. In this sublimate fixed 
material it appears under high powers as a very dense neuropil, 
more dense even than the molecular layer of the cerebellum. It 
fits like a cap over the region of the axone hillock and the adjoin- 
ing medial surface of the cell, the axone passes straight through 
the middle of it,as may be seen in figure 6 taken from a trout larva 
and figure 11 from a young Ameiurus. As has been said, Mauth- 
ner’s cell is turned so that the lateral parts are caudal to the 
medial parts and the axone arises from the superior medial sur- 
face. This is, accordingly, the part of the cell body covered by 
the cap, which extends over the root of the superior ventral den- 
