354 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
each, in front of the hilus where the vessels for the fore-brain enter, 
arises the long slender olfactory tract (Fig. 14). With its neighbour 
it runs along the floor of the brain case near the middle line til] it 
reaches the olfactory lobe which lies directly against the nasal sac, so ~ 
that the numerous olfactory nerves are extremely short. It is only 
recently that Rabl-Riickhard has pointed out that each olfactory tract 
and lobe is a hollow outgrowth of the secondary fore-brain, carrying 
with it a process of the ventriculus communis. Each tract instead 
of being a solid cord is in fact a tube, the roof and sides of which 
are extremely thin, while the floor is so thickened as nearly to fill 
the cavity of the tube. In young specimens where the olfactory tract 
is extremely short and the olfactory lobe still lies close to the cerebral 
hemispheres it is easy enough to demonstrate this, but it becomes 
more difficult to do so in the adult, when the tracts have become 
much elongated. 
From the dorsal aspect it is impossible to see anything of the 
primary forebrain or thalamencephalon, for both it and the medial por- 
tion of the roof of the midbrain are covered by the. great impair 
cerebellum (CB), which, in fact, partly overlaps the cerebral hemi- 
spheres. At each side of the cerebellum, however, are to be seen 
the lateral parts of the midbrain, the optic lobes (ZO), which in 
accordance with the small size of the eyes are themselves very small. 
Behind these the cerebellum is continuous by its postero-lateral angles 
with the tubercula acustica, which are themselves joined behind the 
cerebellum by a bridge of gray matter which roofs over the fourth 
ventricle in front of the trigeminal lobes. The great size of the 
cerebellum, its direction forwards so as to overlap the forebrain, and 
the great size of the twhercula acustica are prominent peculiarities of 
the brain of Amiwrus. In accordance with the great size of the 
fifth and vagus nerves, the lobes of the medulla oblongata in which 
these take origin are proportionately large. They project from the 
floor of the fourth ventricle, so as to leave merely an irregular 
sagittal slit in place of the usual rhomboidal groove. Of the two 
pairs of lobes, the anterior or trigeminal (Z7') are the larger, and 
one of them not uncommonly projects beyond the middle line so as 
to encroach on that of the other side. No fusion ever takes place, 
as is the case with the Cyprinoids, so that there is always the slit-like 
fourth ventricle between the trigeminal lobes of Amiwrus, whereas 
in the Cyprinoids they are coalesced into one lobus impar. The 
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