42 
BULLETIN OF THE LABORATORIES 
illustrates this commissure in its middle course slightly dorsad to Fig. 
lo. The cephalic crus of the epiphysis passes up from the highest 
point of this commisure. In Erethizon this commisure seems to be 
stronger and lies well up in the stalk of the epiphysis, the fibres arch- 
ing much higher than in Geomys. 
The second commissural system caudad of the ventricle of the 
epiphysis, is represented by the posterior commissure in the caudal crus 
of the epiphysis and the commissure of the optic lobes Sylvian com- 
missure in the substance of the corpora quadrigemina dorsad of the 
aqueduct of Sylvius. The posterior commissure is a very strong 
band forming the roof of the third ventricle below the epiphysis, 
and also arching up into the caudal crus of that body. Its fibres 
pass ventrad and caudad into the substance of the optic lobes. 
The commissure of the optic lobes consists for the most^part of 
sparse fibres passing latero-caudad into the dorsal portions of the 
optic lobes. At its cephalic end, fibres dip directly ventrad, arch- 
ing slightly caudad, then cephalad in the substance of the tegmentum 
to a point cephalad of the peduncular nidulus. It is not a strong 
commissure and its fibres do not lie in immediate contact with the 
epithelium of the ventricle, as do those of the posterior commissure. 
Cephalad, the commissure of the optic lobes, seems to be almost, 
if not quite, continuous with the posterior commissure, being dis- 
tinguished from It by its fewer fibres, its position removed from the 
ventricle, and the direction of its fibres. In the opossum, how- 
ever, the continuity is distinctly broken dorsad of the cephalic end 
of the aqueduct. Figs. 9 and 14 of Plate II illustrate the posteror 
commissure; Fig. 10, the cephalic fibres of the commissure of the 
optic lobes; Fig. ii, the sparse fibres of this commissure in the 
middle portion of its course; Fig. 12, its caudal end, the fibres con- 
necting the caudal members of the corpora quadrigemina. It is 
stronger and more compact here than any where else in its course. 
Cranial Nerves. The internal course of the nerve roots, so far as 
observed, conforms in general to the usual descriptions. The course 
of the third is beautifully shown by transverse sections of Geomys 
(Plate III, Fig. 5.) Its nidulus lies immediately dorsad of the dorso- 
median fasciculus, the fibres of which pass through its lower portion. 
It IS very small and sharply defined. The fibres passing down arch 
laterad and pass through the median portion of the large peduncular 
nidulus. The cells of the latter are very large and multipolar, being 
