362 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
giving place to molecular substance especially near the ventricle, 
(Figs. 9 and 10). Itis much simpler in its form than the valvula of 
the Cyprinoids, as may be judged from the sections: its anterior 
tip lying between the tort longitudinales is formed solely of cortex. 
One of the most characteristic features of the brain of Amiurus is 
the forward growth of the cerebellum itself. Becoming independent 
of the valvula in a plane between those represented on Figs. 8 and 9, 
it projects forwards as far as the plane of the commisswra anterior 
(Fig. 19). (In its free part which thus overlies the roof of the mid- 
brain as well as the thin roofs of the intermediate and fore-brain, the 
molecular substance is always completely invested by cortex. 
The great development of the hind-brain of Amiwrus is associated 
with a comparatively small mid-brain, which only reaches the free 
surface in the form of the optic lobes. It is easy enough to deter- 
mine the boundary between mid-brain and thalamencephalon ; it is 
formed by the fusion of the tori longitudinales with the commissura 
posterior. Mayser selects, with other authors, the decussation of the 
fourth nerves as the boundary between mid- and hind-brain. The 
boundary between the parts formed from the second and third 
cerebral vesicles is more difficult to determine in the adult, owing to 
the manner in which the valvula cerebelli is projected forwards into 
the mesocoele (ventricle of the mid-brain), but it is to be understood 
that the lateral cornua of the mesocoele (ventricles of the optic lobes), 
[VZO], and consequently their walls, which form the lateral parts of 
the mid-brain, are to be found both in front of (Fig. 14) and behind 
(Fig. 8), the aquceductus Sylvii and its walls, which constitute the 
central part of the mid-brain. The lateral walls and roofs of the 
ventricles of the optic lobes are everywhere formed by the tecta optica, 
while the medial walls and floors are formed by the tort semicircu- 
lares. Penetrating the ventricles and thus effecting a union between 
the tori semicirculares and tecta optica are the radiating ‘ Stabkranz’ 
fibres. (Radiatio thalami of Fritsch.) A comparison of Figs. 8 to 
14 will show the course of the tori longitudinales. At first hardly 
projecting into the internal and upper angles of the ventricles of the 
optic lobes, they gradually become more prominent. In the more 
posterior planes separated widely by the valvula cerebelli, they con- 
verge till, at the plane of the commissura posterior, (Fig. 13), they 
are almost in contact. Immediately behind that the central part of 
the roof of the mid-brain is formed simply of transverse fibres trace- 
