62 "Journal of Comparative Neurology and Psychology. 



Finally, I must demonstrate the origins of the eye-muscle 

 nerves. This I have already done when treating of the fasc. 

 long, dorsalis for the A^. oculo^notorius whose roots arise, about 

 half crossed and half uncrossed, from a mass of rather large polyg- 

 onal cells situated behind the nucleus fasciculi long, dorsalis 

 near the medial line under the aqueduct (Figs, xxxi and li). The 

 fibers run downward and somewhat laterally and leave the brain 

 in the fissure formed by the lobi inferiores and the base of the mid- 

 brain. 



A nucleus caudad from this situated in the same region and 

 composed of the same kmd of cells as those just mentioned is the 

 nucleus of the ISf. trochlearis, which, in Lophius sends its fibers 

 around the aquaeductus and then in a single bundle almost per- 

 pendicularly upward (Fig. xxxiii). In Gadus, on the contrary, 

 it runs obliquely upward through the velum in two bundles, of 

 which the upper lies over and the lower under the tr. mesen- 

 cephalo-cerebellaris superior which enters there (Figs. Hi, Ixxxvi, 

 Ixxxvii). The most dorsal of these roots leaves the upper side 

 of the velum between the cerebellum and the most posterior part 

 of the tectum opticum and then unites with the lower root which 

 leaves the brain separately. Thereupon they unite and pass 

 forward, for a short distance accompanying the trigeminus. 



2. The Mid-hrain of the Selachians. 



The structure of the mid-brain in the selachians, though in the 

 main the same as that of the teleosts, differs by the lesser develop- 

 ment of the tectum which does not overlap the adjacent parts of 

 the brain as much as in bony fishes. Under the tectum, however, 

 there is a greater difference in the structure of the sub-tectal mass, 

 due to the absence of the torus longitudinalis and the very much 

 smaller development of the colliculus (torus semicircularis), 

 which has not been mentioned by any of those who have made 

 the selachians the subject of their investigations. Catois sup- 

 poses the analogue of the torus to be situated in the more central 

 layers of the mid-brain. Haller also mentions that the torus 

 semicircularis in the teleosts is a specially developed region, and 

 neither Houser nor Edinger mention it for the Plagiostomata. 

 As in the bony fishes, the posterior part of the mid-brain is chiefly 

 a passage for tracts; but in the most anterior dorsal part imme- 



