40 BULLETIN OF THE LABORATORIES 



eiim in 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, /. 



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 out with especial distinctness. The optic tract passes 

 laterad from the 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 prsegeniculatum, 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 Medicommissurc, 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 medicommissure 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. 



Habena. 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 cms 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 



