92 Journal of Comparative Neurology and Psychology. 



Contrary to my expectations, I could trace here the different 

 components of the fasciculus longitudmalis posterior or dorsalis 

 almost as well as in the teleosts. There is no doubt at all about 

 the first group of fibers which join this fasciculus after its origin 

 in the mid-brain, the decussated nerve roots. The entrance of 

 motor fibers into the fasciculus may be demonstrated in selachians 

 as clearly as in other animals. This is seen for the motor trigem- 

 inus in Fig. ci, and for the motor facialis in Fig. ciii. It follows 

 from the whole theory of the coordination of eye movements that 

 there must be other fibers in addition to these short ones, in this 

 fasciculus to effect this coordination, and it has been proved by 

 others that there are fibers coming from the motor nuclei which 

 continue their course in the dorsal longitudinal bundle a great deal 

 farther than the root fibers. These components, however, form 

 only a part of the tracts which appear in this complex and do not 

 sufficiently explain the fact which is so striking in the selachians 

 that where the lobus nervi lateralis anterioris appears the fascic- 

 ulus longitudinalis posterior assumes greatly increased dimen- 

 sions [cf. Figs, cii and ciii, Plate VII). This augmentation may 

 be attributed in part to the entrance of motor trigeminus fibers, 

 but more to the entrance of quite different tracts which run for a 

 much greater distance in the fasciculus. 



In Figs. Ixx, Ixxi, Ixxii, Plate IV, we see, after the tr. tecto- 

 cerebellaris superior has ascended along the inner side of the cere- 

 bellar arm, immediately behind this tract fibers descending along 

 almost the same course (marked 55) running backward in a longi- 

 tudinal direction adjacent to the lateral walls of the ventricles. 

 But as the configuration of the cerebellum changes by reason of 

 the formation of what is generally called its under lip this tract is 

 pressed downward, becomes more compact and more mesially 

 placed under the ventricle (Fig. ci, Plate VII) and where finally 

 the lateral wall is still more reduced by the beginning of the lobus 

 nervi lateralis anterioris these fibers are pushed still farther toward 

 the median line, while the most medial of them enter the fasciculus 

 longitudinalis dorsalis whose dimensions are thereby increased. 

 In the following sections (Figs, cii, ciii) this is still more the case 

 since now, even before the sensory facialis has occupied the place 

 which thev formerly occupied, all of the fibers have entered the 

 upper half of the fasc. long, dorsalis, which has, moreover, been 

 augmented by fibers originating from the whole octavus region. 



