180 WILLIAM F. ALLEN 



root is made between the trigeminal motor and sensory nuclei, 

 it will be found to contain many degenerated ascending mesen- 

 cephalic root fibers (Mes.V.). These fibers are central processes 

 of semilunar ganglion cells, now leaving the trigeminal sensory 

 root to enter the mesencephalic root, which at this level is mixed 

 with the trigeminal motor fibers. In another series (ex. 51) 

 where the dorsal half of trigeminal sensory root was cut, no 

 degenerated fibers appeared in the mesencephalic root. This 

 would indicate that the ascending sensory mesencephalic root 

 fibers came from the mandibular, rather than from ophthalmic 

 and maxillary portions of the trigeminal nerve. The importance 

 of this observation will be considered under the discussion of the 

 mesencephalic root at the end of the paper. A still more caudal 

 section (fig. 5), passing through the caudal end of the trigeminal 

 motor nucleus (M.F.), porti-ays many large and small degener- 

 ated fibers in the mesencephalic root (Mes.V.), which will be 

 seen extending obliquely between the ventral border of the 

 trigeminal sensory nucleus (Sub.G.) and the locus coeruleus 

 (Loc.C). It passes directly lateral to the trigeminal motor 

 nucleus (M.V.) and sends to it, many fibers and fine collaterals. 

 A little below the locus coeruleus it also sends fibers and col- 

 laterals to a group of small cells situated median and dorsal 

 to the trigeminal sensory nucleus. A portion of the mesen- 

 cephalic root located above and lateral to the trigeminal motor 

 nucleus in this section is shown more highly magnified in figure 6 

 so that every particle of degenerated myelin of any size could 

 be accurately sketched. Observe the coarse degenerated fibers 

 and fine collaterals in the mesencephalic root (Mes.V.), in the 

 trigeminal motor nucleus (M.V.), and to the left of the mesen- 

 cephalic root. A corresponding view of the right mesencephalic 

 root (fig. 7) from the opposite or non-lesion side, shows no more 

 degenerated fibers than would be found in any other area of this 

 section not affected by this lesion or in a similar section from a 

 normal brain. 



It is clear from figures 5, 6, and 8 that the great bulk of the 

 ascending mesencephalic root fibers, taking origin from cells in 

 the semilunar ganglion, have ended in the trigeminal motor 



