Houser, The Neurones of a Selachian. 129 
from a polygonal cell-body. The office of this neurone may 
be to bring the collateral branches of the other and more nu- 
merous nervous elements into relationship. Fig. 54 shows a 
group of five of these cells, an unusually large number to find 
in so limited a field. 
2. The Central Gray Matter. 
The aqueduct of SyLvius is surrounded by a layer of nerve- 
cells and nerve-fibres representing the most archaic part of the 
brain, and quite properly termed the central gray matter (Figs. 
20 and 21, ¢. g. m.). The greater part of this nervous matter 
has been supplanted or overshadowed functionally by more re- 
cent additions upon its outer surface, but certain well defined 
groups of neurones have retained their pristine importance 
through the character of their ultimate connections. These 
are, respectively, the roof-nucleus, and the nuclei of the oculo- 
motorius and the trochlearis. 
a. The Roof-Nucleus.—This isa collection of very large 
neurones lying in the roof of the aqueduct of Sytvrus between 
the ependyma and the dorsal commissure of the stratum med- 
ullare profundum. The group has a considerable longitudinal 
extension, reaching from the anterior end of the optic lobes to 
near the juncture with the cerebellum. It is broken into two 
lateral halves by the median line. Fig. 20, ~ z., exhibits the 
entire nucleus ; while Fig. 55 represents the distribution of the 
cells more in detail on the right side. 
These neurones are the largest of any in the nervous organ- 
ization of Mustelus. The size attained by the cell-body may 
be as great as 60 y in transverse diameter by 100 yp lengthwise. 
The group, therefore, presents avery striking picture in the 
field of the microscope. The forms assumed by the cell-bodies | 
are somewhat diverse, ranging from oval, through irregular out- 
lines to a considerably elongated condition. The longer axis 
lies parallel, or nearly so, with the limitans interna. 
In a thick section, the dendrites appear as two or three 
stout processes which push their way into the nervous matter 
dorsal to the group and are soon lost to view. Those cells 
