190 WILLIAM F. ALLEN 



light area designated as the motor root (M.R.V.) in figure 11 is 

 identical to the motor root in figure 1 9 . Also the area of degenerated 

 trigeminal motor fibers in figure 28 (M.R.V.), and better shown 

 in more proximal sections of series 65, conforms exactly to the 

 motor-root area of figure 19. A more detailed discussion of 

 these roots is given below. 



These results are in direct confirmation of May and Horsley's 

 conclusions that the descending mesencephalic root fibers enter 

 the motor root of the trigemmal nerve, and are opposed to John- 

 ston's observations that they entered the trigeminal sensory 

 root. Since Johnston worked only with normal brains stained 

 after the Weigert method, he could have easily confused the 

 descending mesencephalic root fibers with ascending sensory 

 fibers entering the mesencephalic root, which are very numerous 

 at this level (see previous description and fig. 8, Mes.V.). 



The descending mesencephalic root fibers, like the ascending, 

 send off collaterals to the trigeminal motor nucleus and to a 

 group of cells situated a httle median and dorsal to the trigeminal 

 sensory root (figs. 15, 17, and especially 16). In the last- 

 mentioned figure a portion of the mesencephalic root and the 

 motor nucleus of the trigeminal nerve are shown more highly 

 magnified to permit of an accurate drawing of every particle of 

 degenerated myelin. A comparison of this figure with a similar 

 drawing of the ascending fibers of the mesencephalic root (fig. 6, 

 Mes.V.) shows at least twice as many descending mesencephalic 

 root fibers in figure 16, but fewer fibers and collaterals (Col.) 

 are given off to the trigeminal motor nucleus. An examination 

 of the right mesencephalic root (non-lesion side) in this series 

 demonstrates no more degenerated fibers than would be present 

 in similar sections of a normal brain. 



If an examination of the left trigeminal roots is made about 

 half way between the exit of the motor root and the beginning 

 of the semilunar ganghon (fig. 20), it will be seen that both roots 

 are surrounded by the same connective-tissue sheath, which also 

 contains the trochlear nerve (IV). The latter possesses many 

 degenerated fibers and was undoubtedly severed, centrally, in 

 making the lesion of the mesencephalic root. The motor root 



