104 JOURNAL oF COMPARATIVE NEUROLOGY. 
8. Supporting Elements. 
a. Ependyma.—The ependyma of the oblongata constitutes 
the membrana limitans interna throughout, of course, but the 
degree of differentiation covers a wide range. The simplest 
condition is found in the tela choroidea, for the cells here do 
not give origin to ependymal fibres. This might be called the 
epithelial phase of development. 
The first step in the differentiation of ependyma occurs in 
the dorso-lateral region of the oblongata, embracing the tuber- 
culum acusticum and the adjacent part of the general cutaneous 
nucleus. The ependymal cells are rather narrow here, giving 
a larger number per surface area. From the tip of each cell, a 
very slender ependymal fibre pushes its way through the ner- 
vous matter to the limitans externa. The fibre does not branch, 
and its general course is a straight one, although the details of 
its path are very irregular indeed, sharp turns of small size oc- 
curring along its whole length. While the diameter of the 
fibre as a whole is quite slender, its size is increased at irregular 
intervals by beads and small knobs. Consult Fig. 8. 
A slightly more advanced condition is to be noted for the 
ependyma of the ventral region. The ependymal cells in the 
floor of the fourth ventricle are larger than those described 
above, and their fibres are also much stouter. The general 
course of these fibres is shown in Fig. 2, ef.; and the details of 
two fibres are given in Fig. 9. It is to be noted that the fibre 
extends entirely to the limitans externa, that it bears no 
branches, that its course is free from sharp turns, and that its 
diameter is varied by nodes and thickenings of various shapes 
and sizes. Groups of these fibres form radiating bands through 
the formatio reticularis, and their evident purpose is to provide 
stays for the vast number of longitudinal and arcuate nerve-fibres 
which characterize this part of the oblongata. 
The ependyma attains its highest differentiation in the 
lobus vagi; see Fig. 10. The fibres do not reach the surface, 
and they give origin to a most complicated series of branches 
at the level occupied by the neurones of the lobus. The prox- 
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