362 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



giving place to molecular substance especially near the ventricle, 

 (Figs. 9 and 10). It is much simpler in its form than the valvulaoi 

 the Cyprinoids, as may be judged from the sections : its anterior 

 tip lying between the tori 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 commissura anterior 

 (Fig. 1.9). 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 Aviiurus 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), 

 [7X0], 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 aquozductus 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 tori 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- 



