354 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



each, in front of the hilus where the vessels for the fore-bi^ain 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 till 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-Ruckhard 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 (LO), 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 tubercula acustica are prominent peculiarities of 

 the brain of Amiurus. 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 {LT) 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 Amiurus, whereas 

 in the Cyprinoids they are coalesced into one lobus impar. The 



