Houser, Zhe Neurones of a Selachian. 81 
glossus, these nuclei disappear altogether as continuous collec- 
tions of nerve-cells. A small number remain associated as the 
nucleus of the abducens, while the remaining neurones become 
scattered through the formatio reticularis (Fig. 2, c. x. and ¢. x.). 
The intermediate zone and the dorsal cornu of the cord 
are relatively small in size, but as the paired halves diverge to 
right and left of the fourth ventricle they increase in mass and 
assume characters and functions of a special order for each 
region. The intermediate gray matter contributes the lobus 
vagi and the viscero-motor nucleus of the oblongata; while the 
dorsal cornu becomes specialized as the general cutaneous nu- 
cleus. We will note each of these in turn. 
The lobus vagi is a longitudinal elevation in the lateral wall 
of the fourth ventricle (Fig. 1, 4. ug.). Anatomically, it is one 
of the most striking features of the oblongata because of the 
row of bead-like prominences into which its surface is thrown. 
The position of the structure as seen in a transverse section is 
represented in Fig. 2, 7. ug. The lobus vagi is the terminal 
station for communis components of the VII, LX, and X nerves; 
see Subsection 4. Certain of these fibres, instead of passing 
‘directly to their termination here, enter a compact bundle and 
run posteriorly to the spinal cord. This tract is known as the 
fasciculus communis. Its position in the oblongata will be seen 
A PAS 2 Farid 
The viscero-motor nucleus is a column of large nerve-cells 
imbedded in the lateral wall of the fourth ventricle (Fig. 2, v. 
m. n.). Different portions of this column are known as the 
nucleus ambiguus, and the motor nuclei of the VII and V, res- 
pectively. The axones for the motor roots of the V, VII, IX, 
and X nerves take their origin from these nerve-cells. It is a 
curious fact, however, that but few of the axones pass directly 
into their nerve-roots, but take a course first in the fasciculus 
longitudinalis dorsalis. This is a massive, paired tract, the two 
bundles lying side by side beneath the floor of the fourth ven- 
tricle (Fig. 2, f. 7. d.). There are present in the dorsal longi- 
tudinal bundle nerve-fibers from several sources. Into this 
crowded highway the axones from the viscero-motor nucleus 
