No. 1.] AMPHIBIAN BRAIN STUDIES. 75 
nerve which decussate to the opposite side of the brain through 
the cerebellum, and either enter the main sensory tract or ter- 
minate in some of the cells of the mesencephalon. The de- 
scending Trigeminus tract does not decussate, but enters the 
-medulla on the same side. 
The fine fibres of the Urodele cerebellum are 
in part decussating tracts of the Auditory nerve, 
and the coarse fibres are non-decussating descend- 
ing tracts of the Trigeminus nerve. 
This conclusion as to the connection with the Auditory 
nerve is supported by Ko6ppen’s observations upon Raza (op. 
cit., p. 13), and Ahlborn (p. 261). Ahlborn, however, considers 
that the fibres entering the cerebellum are commissural between 
the Auditory nuclei of opposite sides. This hypothesis is not 
supported by the fact that the superior and inferior tracts are 
subequal in diameter. 
The passage from the medulla into the mesencephalon is 
accompanied by the compact disposition of the gray substance 
immediately surrounding the ventricle, Fig. 25. The fine fibres 
descending to the trigeminus and the great mass of fibres 
from the central region of the medulla become indistinguish- 
able, and the only fibres which can be readily followed forwards 
in transverse sections are those of the descending trigeminal 
tract and the posterior longitudinal fasciculus. Anteriorly, in 
the region of the diencephalon, the so-called round bundle is 
sharply differentiated from the remaining fibres. In vertical 
sections, however, the tracts from the central region of the me- 
dulla can be followed with great ease, especially in the brains of 
Amphiuma, Necturus, and Cryptobranchus, in which the mid- 
segments are only slightly swollen and the sections fall in the 
plane of large bundles of fibres (see Fig. 30). These tracts have 
been carefully studied in Mecturus, Cryptobranchus, and Rana, 
the former genera agreeing closely in all essential features. 
Tue NuUCLEI OF THE ANTERIOR SEGMENTS. 
The main masses of fibres from the spinal cord and central 
region of the medulla sweep forwards either directly into the 
central gray of the mesencephalon, diencephalon, or prosen- 
