106 J. B. JOHNSTON, 
at either side of the mid-dorsal line, a collection of very large cells 
irregularly grouped in one or two layers bordering on the central 
cavity. These cells are never impregnated in GOLGI preparations, 
so that it is impossible to study their dendrites satisfactorily, or to 
determine whether they have neurites. A few of the cells are found 
as far caudally as the middle of the tectum, but the great majority 
are situated dorsal to the posterior commissure, surrounding a blind 
pouch of the aqueduct which extends forward above that commissure 
(Phot. 22). The cell body is usually pear-shaped with a single large 
process from the smaller end. There may be two or more smaller 
processes from other parts of the cell. The smaller end of the cell 
is usually turned away from the cavity and the large process arising 
from this end of the cell turns at once laterally and can be traced 
for some distance in haematoxylin sections as a very thick fibre 
running just over the ependyma cells and among the nerve cells of 
the inner zone. 
2, Fibre Tracts in Tectum. 
I shall defer the description of these tracts until after the 
‘tween brain nuclei have been described and shall take up all the 
tracts of the mid and ’tween brain in one section (see page 124). 
b) Side Wall and Base of Mid Brain. 
1. Central grey Matter. 
The side walls of the mid brain are so filled up with fibre 
tracts that the grey matter is reduced until it is comparatively un- 
important. The cells have much the appearance of the tract and 
commissural cells in the medulla and, I believe, are to be directly 
compared with them. There is, however, a large nucleus probably 
belonging to the central grey matter, which serves as the place of 
ending of the bundles of MEYNERT. I shall here describe the cells 
of this nucleus, leaving the tracing of the bundles of MEYNERT and 
the proof that they end in this nucleus until I take up the detailed 
description of the fibre tracts (see page 130). The nucleus lies near 
the central cavity dorsal to the commissura ansulata. It extends 
from the mid-ventral groove, ental to the fasciculus longitudinalis 
posterior, laterally to near the point of connection of the valvula 
with the side wall of the mid brain. Its cephalic border is clearly 
defined and lies a short distance caudal from the cephalic border 
of the commissura ansulata. It extends caudally to a point some- 
