96 
GEOEGB E. NICHOLES. 
the venti'icular caual. It is lined by a very ordinary short 
columnar epithelium. 
In the thickened roof of the fourth ventricle there is, how- 
ever, a chain of small, discontinuous and somewhat irreguhir 
spaces lined by a flattened epithelium, which continue bnck- 
wards in the middle line directly dorsal to the fourth ventricle 
(Text-fig. 7). The most anterior of these spaces is, in one 
specimen, barely separated from the anterior dilated portion 
of the fourth ventricle by an epithelial wall. 
Frequently these small spaces are divided vertically by a 
nearly median partition, and then appear as paired cavities 
lying on either side of the middle line. In one case there was 
a median space with a small lateral chamber on either side. 
It is deal- that collectively they represent the vanishing 
vestiges of the dorsal portion of the fourth ventricle. I was 
able to make them out most satisfactorily in a series of sections 
cut transversely. In other series, cut sagittally, they are 
much less conspicuous. 
In every brain examined, however, there was to be 
observed towards the hinder end of the medulla oblongata., 
at the level of these vestigial ventricular spaces (and therefore 
well above the continuous tubular portion of the fourth ven- 
tricle), a considerable space. This, which is to be regarded 
merely as an enlarged member of the dorsal chain of small 
spaces, is in wide communication below with the fourth 
ventricle in one specimen only (Text-fig. 7). In the three 
I’emaining specimens it is separated from the fourth ven- 
tricle by a thin layer of epithelium. Just behind the 
communication between the two cavities (where such com- 
munication occurs) the upper space tapers oS into a narrow 
canal, which runs backwards dorsal and parallel to a similar 
backward continuation of the ventricular canal, the two 
giving rise to the double canal so characteristic of the spinal 
cord of the Myxinoids. I look upon the dilatation of the last 
member of the upper discontinuous series of spaces as indi- 
cating approximately the hinder end of the medulla 
oblongata. 
