264 H. W. NORRIS 
by three to six rootlets from a nucleus of large cells in the ventro- 
lateral gray matter. (38) Radix mesencephalica trigemini. As 
the nature and origin of this root of the fifth nerve have received 
considerable attention of late (Johnston ’09; Bindewald ’11; van 
Valkenburg ’11) a somewhat detailed account of its course in 
Siren will be given. Kingsbury (795, p. 190) and Osborn (’88, p. 
69) both describe a mesencephalic constituent of the trigeminus 
in Necturus and Osborn in Cryptobranchus. In Amphiuma the 
writer (’08, p. 531) has recognized a tract of fibers from the mesen- 
cephalic roof which appears to contribute to the fifth nerve. 
Sections through the entrance of the trigeminal nerve into the 
medulla in Amphiuma show a tract of fibers running antero- 
medially through the acusticum nearly to the ventral border of 
the gray matter, then dorsally and laterally through the posterior 
border of the lateral wall of the midbrain into the tectum. Of 
the course of the radix mesencephalica V in Necturus, Johnston 
(05 a; p. 370) says: 
When the junction of the cerebellum and tectum is reached the 
tract has collected into a compact bundle, which is imbedded in the 
thickness of the brain wall. The bundle now turns dorsally and divides 
into mesial and lateral parts. The lateral part is finer fibered. It 
arches up around the lateral lobe of the cerebellum close to the junction 
with the tectum and forms a commissure in the dorsal wall of the cere- 
bellum which in Necturus lies forward over the tectum opticum. 
On superficial examination this description seems to answer 
for the conditions in Siren, but after careful study the relations as 
above described are seen to be considerably modified. The radix 
mesencephalica V in Siren enters the brain (usually) in two 
tracts of coarse fibers (figs. 20, 24, Vrm.). The more posterior 
of these divides, one division passing anteriorly and joining the 
Figs. 20 and 21 Sagittal sections through the auricular lobe of the brain, 
showing the internal distribution of the radix mesencephalica V, and the relations 
of the trochlear nerve to the radix and to the cerebellar commissure. 50. Com- 
pare with figures 22 to 27. 
Fig. 20 Shows the division of the radix into anterior and posterior parts as 
it enters the brain. 
Fig. 21 Three sections lateral to the preceding. The relation of the radix to 
the tractus tecto-bulbaris et spinalis is shown. The roots of the facial nerve are 
seen. 
