Houser, Zhe Neurones of a Selachian. 105 
imal part of the fibre is quite stout and somewhat irregular 
in its course. The distal part becomes dissolved in branches, 
the terminal twigs of which are minute. The interlacing 
branches form a support for the neurones and nerve-fibres 
proper to this region. 
b. Neurogha.—The neuroglia of the oblongata is, like the 
ependyma, present in great diversity of form, but the various 
elements are referable to two general classes. One, the well- 
‘known astrocyte, is distributed in every part of the oblongata 
where there are nerve-fibres in numbers—in the roots of nerves, 
in special tracts, in centres where many fibres have their termin- 
ation, and in the formatio reticularis. The special form of the 
astrocyte depends, to some extent at least, upon the density 
with which the nerve-fibres are arranged. At points where 
there has been little pressure, the rounded form of the cell- 
body may be retained (Fig. 3, zg.). . In certain closely packed 
tracts, the cell is squeezed into lenticular form. In centres where 
many minute fibres interlace, there may be hardly any body to 
the cell at all, the matter being chiefly in the radiating proc- 
esses (Fig. 11). In every instance, the processes exhibit tor- 
tuous irregularities due to their insinuation between the nerve- 
fibres among which they lie. 
Another type of neurogliar cell appears to be characteristic 
of the ventral part of the oblongata, occurring in some numbers 
in the formatio reticularis (Fig. 3). The cell-body is large, 
elongated, with great processes emerging from its ends anda 
few smaller ones from its sides. The large processes take a 
course in the radius of the oblongata. They give origin to 
secondary and tertiary branches, the finer ones of which lie at 
right angles to the course of the principal branch. The whole 
system does not spread widely, but has the appearance of com- 
pression in one plane. 
The astrocyte is evidently an important supporting element 
for the individual nerve-fibres between which its processes 
twine. The second class of neurogliar cell described seems to 
be adapted to the supporting of bundles of nerve-fibres. The 
arcuate fibres of the formatio reticularis pass in groups through 
