LATERAL SENSE ORGANS OF SQUALUS SUCKLII 283 
The peripheral terminations of the lateral nerve. On reaching 
the base of the sensory column the fibers of the lateral ramuli 
diverge caudad and cephalad in the subbasilar fiber zone. This 
fiber zone is shown in longitudinal section in figures 2 and 4, and 
in transverse section in figures 1 and 3. The majority of the 
fibers are medullated but a few non-medullated fibers can be 
found. These can be traced back through the ramulus to the 
lateral nerve, which indicates that they are not simply non- 
medullated branches of the large medullated fibers. 
Two zones of distribution or branching of the nerve fibers 
appear well marked. Primary distribution takes place from the 
longitudinal fiber zone and the branching is almost entirely 
subbasilar (figs. 4 and 10), while a secondary zone of distribution 
or branching is located roughly between the limits marked by the 
-nuclei of the basilar cells and the proximal ends of the hair cells. 
It is from this zone that the fine fibrillae arise which pass out 
freely between the hair cells. 
The primary branches are large and coarse as a rule (fig. 7), 
although many fine branches arise from this zone also (fig. 4). 
Branching of the fibers appears frequently to be dichotomous but 
not uncommonly three or more branches arise at the same level. 
This statement holds for both zones of distribution. Enlarge- 
ments of considerable size are commonly seen at the level of 
branching of the nerve fibers (fig. 9), but it seems likely that the 
majority of these extra large ‘‘varicosities’”’ are caused by an 
over-deposit of silver at the points of branching. One or more 
fibers may rise from the subbasilar fiber zone to supply a single 
cluster of hair cells, and occasionally the fibrillae of a given fiber 
ramify in adjacent groups of hair cells (fig. 10). The medullary 
sheath is usually lost just outside of the basilar membrane. 
The primary branches rise to a considerable height in the 
sensory epithelium—usually beyond the nuclei of the basal cells— 
where they form a rather rich plexiform network (figs. 4, 7, and 
10). This network forms the secondary zone of distribution and 
it is from it that the ultimate distribution of fibrillae to the hair 
cells takes place. While this secondary zone of distribution is 
present in the lateral sensory epithelium of Mustelus canis, it is 
