50 
GEORGE E, NIGHOELS. 
These veinain distinct for a considerable distance through the 
brain cavity, uniting, in a manner shortly to be described 
(hg. 11, to form a single median structure only beueatli 
the posterior part of the tectum mesencephali, imme- 
diately in front of the point where that structure joins the 
cerebellum (Text-lig. 5). 
The anterior surface of the posterior commissure, over 
which the paired ependymal gi-ooves pass, has a presentation 
approaching the vertical (see Text-fig. 5), but slopes slightly 
backwards. The course of the paii-ed fibres, therefore, is at 
first steeply downward and slightly backward. Moreover, 
they also incline somewhat towards the middle line, and it is 
thus not possible (in this species) to obtain any considerable 
length of the fibre in this region in any one (thin) section cut 
in any of the three conventional planes. 
In transverse sections especially the fibre is, in this 
region, cut very obliquely, and, as it lies in this part of its 
course closely against the ependymal epithelium, it frequently 
may, by focussing, be seen apparently penetrating this 
epithelium. Really, of course, it simply lies against the 
ependymal epithelium upon its free surface (fig. 38, r. /.). 
Lying close adjacent to the ventral surface of the 
posterior commissure, and separated from the aqueductus 
Sylvii only by that flattened epithelium which extends 
mesially between the two grooves of the sub-commissural 
organ, are numerous conspicuous nerve-fibres. These are the 
axons of the large, laterally situated cells (fig. 36, n. 2^. c.), 
which .Johnston (’02) has identified as the nucleus of the 
posterior commissure, and they are the nerve-fibres which 
form, according to Sargent (’04, pp. 154-155, pi. i, figs. 6 
and 7), the second of the three sources from which he would 
derive Reissner’s fibre in the lamprey. Owing to the 
extremely flattened character of the ependymal epithelium 
here, many of these axons lie quite superficially. Whe’re they 
pass laterally into the region of the sub-commissural organ 
they still continue, for a while, to course superficially between 
the radiating fibre-lihe ependymal cells, presenting occasion- 
