Houser, The Neurones of a Selachian. 157 
slightly different levels, and so a broad zone next to the ven- 
tricle presents chiefly nuclei. The shape of the cell-body is 
influenced by the position of its nucleus, of course, but the 
broadened part is usually not directly in contact with the ven- 
tricle. The ependymal fibre runs straight outward from the cell- 
body, as a rule, and it reaches entirely to the limitans externa. 
It does not branch, its size remains nearly uniform, and its 
course is only slightly irregular. It bears a greater or less 
profusion of delicate, mossy twigs of short length. The inner 
portions of two ependymal elements are shown in Fig. 42. 
Neuroglia is found in the several parts of the forebrain 
where nerve-cells are grouped in numbers. While the specific 
forms assumed by neurogliar elements exhibit considerable 
diversity, all are referable to but one type. From an irregular 
cell-body, numerous fine processes extend in all directions for a 
short distance, branching profusely as they proceed. The ulti- 
mate twigs are of quite minute size, and hence it is that the 
whole presents a characteristically mossy appearance. Fig. 43 
illustrates the features of a representative specimen. 
6. Summary of the Forebrain. 
A fairly well-defined epistriatum is present, receiving olfac- 
tory and other terminations. Its neurones are of the Gorter II 
type, sending their axones into the striatum, in the main. The 
axones from marginal zones enter the pallium. 
Neurones with widely spreading dendrites are arranged in 
open order in the striatum. Their axones enter the tractus 
strio-thalamicus for termination in the thalamus. The striatum 
appears to be an olfacto-motor centre. 
The nucleus postolfactorius is a densely crowded group of 
neurones. Olfactory fibres terminate here, and the derivative 
axones form the tractus olfacto-habenularis. © 
A group of large neurones in the vicinity of the recessus 
neuroporicus gives origin to the median portion of the tractus 
Strio-thalamicus. This paired tract accompanies the neuropore 
for some distance. The nucleus neuroporicus is a third olfac- 
tory centre. 
