the epithelium which lines the grooves to that which lines the adjacent parts of the brain-cavity takes place abruptly, and there are indications on the latter of very much longer cilia than occur in the grooves. As far back as 1873, indeed, Langerhans pointed out in his researches on Petromyzon platwi that the ventricles of the brain are lined with ciliated cylinder-epithelium.*
The upper and inner part of each groove is in this region imbedded in the substance of the posterior commissure ; the lower and outer portion in the mass of cells which forms the inner part of the brain substance (fig. 2). Anteriorly the two grooves diverge from one another on the roof of the recessus sub-pinealis, and disappear in the deep crevices between the ganglia habenulse and the side walls of the brain. Posteriorly they terminate at the hinder margin of the posterior commissure. Immediately behind this point the choroid plexus of the mid-brain dips down into the aquseductus Sylvii, or iter, in the shape of a thin vertical lamella, which forms a highly vascular longitudinal septum dividing the upper part of the iter into right and left halves (fig. 3). The anterior portion of this septum is visible
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in sections beneath the ciliated grooves in the region of the posterior commissure (fig. 1), while it extends backwards throughout the 'whole or very nearly the whole length of the mid-brain.
Thanks to the kindness of Dr. S. J. Hickson, F.R.S., Professor of
- ' Berichte iiber die Verlmndlungen der NaturforscLenden Gcsellschaft zu
Freiburg i. B.,' vol. 6, Heft 3, p. 83.
