40 
BULLETIN OF THE LABORATORIES 
eum ipx rodents, the mammillare is pushed far caudad and, in fact, lies 
ventrad of the commisure of the optic lobes. Thus the ascending for- 
nix tract, passes up in the most caudal part of the thalamus, instead of 
the cephalic part, as figured by Meynert. Plate II, Fig. 2, illustrates 
this ascending tract, lying between the caudal end of the third ven- 
tricle and the descending fornix tract, f. 
Optic Tracts. Enveloping the dorsal and caudal aspects of the 
chiasm is a dense cluster of very large multipolar cells, which in the 
opossum has been called the anterior nidulus of the cinereum. They 
are enveloped in a very loose reticulum of fibres which is quite free 
from cells of other sorts. This causes their strong protoplasmic pro- 
cesses to’ stand with especial distinctness. The optic tract passes 
laterad from thif chiasm to the edge of the thalamus, thence dorsad as 
a compact bundle on its lateral aspect. It passes obliquely dorso-cau- 
dad over the piaegeniculatum, then over the dorsal surface of the post- 
geniculatum, sending small fasciculi of fibres through both of these 
bodies back to the optic lobes. The rest of the fibres pass over the 
dorsal surface of the thalamus, entering the.optic lobe on their cephalic 
aspect. 
The Medicommissiire., or soft commissure, fills nearly the whole of the 
dorsal portion of the third ventricle. As is often stated, it is merely 
an adhesion of the sides of the thalami in the median line ; yet so inti- 
mate is the contact that considerable force is necessary to separate 
them in Geomys, and in Erethizon this could hardly be effected at all. 
Sections, moreover, reveal no internal break in the continuity of the 
substance of the thalami at this place, the meson being occupied by 
cells and fibres like the adjacent parts. In dissecting overhardened 
brains of the rat, however, the medicommissiire some times falls apart 
of its own accord. It is nearly circular in section, in Erethizon 
about 6.5 mm. in diameter, in Geomys, 3 mm. 
Habence. Each habena is a club-shaped body lying on the cepha- 
lo-dorsal aspect of the thalamus. Their larger ends are directed 
caudad and terminate near the median line immediately under the 
cephalic crus of the epiphysis and the superior commissure. The 
smaller ends gradually diverge as they pass into the taenia thalami 
and down over the cephalic face of the thalamus. Microscopically 
the habena consists of two sharply differentiated parts. The median 
portion is very densely packed with small cells which stain intensely. 
This is the nidulus of the habena. The lateral and cephalic portions 
