134 JOURNAL OF COMPARATIVE NEUROLOGY. 
almost invariably triangular. The base of the triangle lies 
toward the nucleus, the apex pointing in the direction of a den- 
drite. Those masses lying in contact with the nuclear mem- 
brane are somewhat broader, taking the form of a so-called 
nuclear cap. The size of mass decreases toward the periphery 
of the cell, those lying in the bases of the dendrites assuming 
a slender form. Reference may be made to Fig. 59. 
The striking size attained by the masses of tigroid sub- 
stance here is doubtless associated with the purely motor func- 
tion of the III nerve, the fibres of which are the axones of 
these particular neurones. 
c. The Nucleus of the Trochlearis.—Rouon (77) fell into 
a curious error with regards the nucleus of the IV nerve. As 
is well known, the root of the trochlearis passes backward and 
crosses over to the opposite side, appearing dorsally in the 
furrow between the midbrain and the cerebellum. ROuHON evi- 
dently sought for the nucleus of the nerve near its superficial 
origin, for he identified as such a group of cells on the border 
of the cerebellum. 
The group of neurones constituting the nucleus of the IV 
nerve lies posterior and slightly ventral to the nucleus of the 
III nerve (Fig. 58, x. JV). The anterior end of the trochlear 
nucleus overlaps the posterior end of the oculomotor nucleus 
for a short distance. 
The cells of this collection are, as compared with the cells 
of the oculomotor nucleus, decidedly smaller in size, and the 
general outline is more nearly triangular. The nucleus of the 
cell is relatively larger in proportion to the amount of cytoplasm. 
The chromatic network is so delicate as to be but faintly visible 
even under high magnification. 
The masses of tigroid substance are few in number, rela- 
tively large in size, and wholly irregular as to form. There isa 
perinuclear zone of cytoplasm entirely free from tigroids. It 
often appears as though many of the tigroid masses are actually 
clinging to the limiting pellicula of the cell. Fig. 60 exhibits 
a condition typical for the cells of this group. 
I really am unable to offer any explanation concerning the 
