CRANIAL NERVES OF SIREN LACERTINA 269 
anterior tract to form the radix mesencephalica V proper, the 
other passing posteriorly at the ventral border of the gray matter, 
(fig. 20, Vrmp.), and can be traced as far posteriorly as the level 
of the root of the seventh nerve (figs. 35-32). The writer’s 
preparations of Necturus indicate a similar arrangement and divi- 
sion of the rootlets forming the radix mesencephalica V. The 
fibers of the radix mesencephalica V proper pass anteriorly, dor- 
sally and medially (figs. 20-27), then posteriorly. As stated by 
Johnston for Necturus, ‘‘when the junction of the cerebellum and 
tectum is reached,”’ the radix mesencephalica V has been joined 
by other groups of fibers, coarse and fine (figs. 21, 22, 25-27, tbs., 
lm.), so that it becomes very difficult to distinguish accurately 
between it and the other fibers. The fibers of the mesencephalica 
V are, however, coarser and more heavily medullated: These 
finer and less medullated fibers that join the radix mesencephalica 
V come in part from a ventro-laterally situated tract of the med- 
ulla which the writer interprets as the tractus tecto-bulbaris et 
spinalis (tbs.) ; some come also from more centrally running tracts, 
possibly the lemniscus system (/m.). These finer fibers evidently 
contribute to the internal part of the radix mesencephalica V and 
in all probability form the lateral finer-fibered division of the 
latter (Vrml.) to be described later. As in Necturus, so in Siren, 
a division of the radix into medial and lateral parts occurs, the 
lateral being finer fibered. The lateral finer-fibered part does 
not, however, arch up around the lateral lobe of the cerebellum 
and form a commissure in the dorsal wall of the cerebellum. It 
divides and one part passes postero-dorsally close to and parallel 
with the cerebellar commissure for some distance (figs. 22, 24-26). 
At the point where the trochlearis enters the latter commissure 
(fig. 27) fibers from the lateral fine fibered part of the radix also 
enter the commissure, but the greater part of the lateral fine- 
fibered portion of the radix passes into the posterior dorsal part 
of the tectum (figs. 20, 24). In some instances this fine-fibered 
part on one side of the brain joins the cerebellar commissure so 
closely as to be indistinguishable from it, but on the other side is 
sharply distinct, contributing to it only a few fibers at the point 
above described. In Necturus, in the words of Johnston (l.c.), 
