No. 1.] AMPHIBIAN BRAIN STUDIES. 63 
decussating processes of the ganglion cells! 5. The remaining 
area, which makes up the main central region of the medulla, is 
composed of the sensory and motor tracts. 
The area of large ganglion cells, as shown in Figs. 10, 11, and 
12, is continuous with the cells of the anterior horn. It is also 
clear that the main motor and sensory tracts which occupy the 
anterior and lateral columns, at the level of Fig. 10, are thrust 
down, with the ascending Trigeminal tract, to form the area 5. 
With the exception of the posterior longitudinal fasciculus, none 
of the nuclei or tracts of the cranial nerves below the 6th pair 
occupy the central region of the medulla. It follows that the 
lateral region of the medulla is exclusively com- 
posed of the eranial nerve tracts and nuclei and 
the central region is composed of the main sen- 
sory and motor tracts of the cord and special sen- 
sory and motor centres. These regions are pretty sharply 
defined from each other. The lateral region above the level of 
the Vagus is bounded by the ascending Trigeminus tracts. 
The Tracts and Nuclet, Plate V. A general description 
of the fibre tracts and nuclei is necessary to introduce the 
special study of the origin of each nerve. In the section 
opposite the 12th nerve, we find the columns of the cord and 
horns of gray matter well defined. Between the posterior and 
lateral columns, at the extreme periphery of the cord, is a small 
bundle, the trigeminus ascendens, 5¢/ On the posterior 
horn is another small round bundle, not hitherto to my knowl- 
edge described, which from its common relations to a number of 
the cranial nerves we may call the fasciculus communis. 
Opposite this, upon the lower side of the posterior horn, is 
another round bundle, which from its relation to the Vagus is 
believed to represent the fasciculus solitarius of Len- 
hossek, fs. Above this section, Fig. 11, opposite the exit of 
X. 4 and 5, is seen a small nucleus contributing to the fibres 
of the fasciculus communis, fcz.; then the nucleus and 
ascending bundle of the goth, then the ascending trigeminal 
tract; below this is the exit of a portion of the fasciculus 
communis, and the nucleus and a root of X. 4. In the centre 
of the medulla is the posterior longitudinal fasciculus, 
which is now a more compact and well-defined tract than in 
1 Connected with the crossing Miillerian fibres, (Ahlborn). 
