100 J. B. JOHNSTON, 
of decussating or commissural bundles in the base of the mid brain. 
These fibre tracts can be described best after I have treated of the 
grey masses contributing to them. 
a) Tectum opticum. 
In the description of the external features of the brain I have 
mentioned the groove which separates the tectum from the cere- 
bellum, the less marked grooves which indicate the lateral limits of 
the tectum, and the deep mid-dorsal groove which divides the tectum 
into corpora bigemina. The cephalic limit of the tectum is marked 
by the posterior commissure and the epiphysis. In frontal sections 
stained with haematoxylin certain points in the structure can be 
made out which will be of topographical importance in the de- 
scription of the minute structure (Phots. 16—22). It is seen that 
the tectum is thin along the line of the dorsal groove and is made 
up of fibres passing from side to side, the dorsal decussation. At 
either side of the dorsal groove the ectal portion of the tectum is 
made up largely of small bundles of medullated fibres cut across 
in frontal section. These are the fibres of the optic nerve. The 
groove which marks the lateral border of the tectum is seen in 
section to be very distinct, and in its cephalic part the tectum is 
sharply separated from the side wall of the tween brain (nucleus 
ruber tegmenti) by a thin septum of connective tissue. Dorsal to 
this groove the ectal portion of the tectum is made up of medul- 
lated fibres which are most numerous near the caudo-ventral angle 
of the tectum. These are mostly fibres from the tectum to the 
hypothalamus, cerebellum and medulla, although there are also a 
few optic fibres in this position. In sections through the caudal 
part of the mid-brain (Phot. 18) there is seen a ridge projecting 
into the central cavity from near the lateral border of the tectum, 
the torus semicircularis Halleri. The remainder of the tectum pre- 
sents what may be considered the typical structure. Bordering upon 
the cavity is a zone consisting of several layers of cells which send 
radial processes out through the thickness of the tectum, giving it a 
radial striation. The inner zone of fibres comprises about one-fifth 
of the thickness of the tectum. The outer four-fifths is largely made 
up of fibres, so that it has a “molecular” appearance. Imbedded 
among the fibres are cells, the number of which is best indicated 
by the photographs referred to. Besides these two zones it is im- 
