40 
BULLETIN OF THE LABORATORIES 
area governing the hind leg. These motor reactions were, in the main, 
crossed as usual, but in several instances similar motions of the muscles 
of both sides resulted when superficial irradiation appeared to be ex- 
cluded. The areas thus roughly mapped in the opossum coincide in 
general with those of the ground hog and we are forced to conclude 
that the crucial sulcus of the opossum is not strictly homologous with 
the fissure so named in carnivora. 
The rodent brain is especially noteworthy because of the 
reptilian simplicity of structure. The smooth, depressed and elon- 
gate hemispheres, protruding and exposed olfactory lobes, and partially 
exposed optic lobes are such characters. The large, highly convoluted 
cerebellum redeems the mammalian habitus of the brain. The convo- 
lutions have definite patterns which may be of generic or possibly or- 
dinal character. The extended flocculus is a rodent character. The 
forward extension of the chiasm and the great obliquity of the optic 
tracts, as well as the distinctness of the thalamus from the axial portion 
of the hemisphere may be regarded as primitive or degraded conditions 
as are the relatively posterior position of the corpus callosum, the dis- 
tinctness of the fornix and its reflation to the hippocampus and the 
great size of the ventricles. In spite of the considerable size of the 
cerejellum, it does not entirely cover the fourth ventricle. The sev- 
eral spinal nerves have a degree of independence greatly facilitating 
their study. 
The comparison with the opossum on the one hand and Carnivora 
on the other is instructive as illustrating the range of variation in struc- 
tures essentially similar. The most marked difference between the ro- 
dents and marsupials consists in the relative importance of the' two 
great commissural systems of the fore-brain. 
In Didelphys the callosum is practically absent and the fibres serv- 
ing the function of the corpus callosum m higher mammals are associ- 
ated with the anterior commissure. Tne structure of this region in 
the opuLim is of great interest. The great mass of transverse fibres 
constituting the commissural system of the fore-brain consists of four 
elements, viz : 
I St. Fibres from the olfactory tract which cross from the pyriform 
lobe to the thalamus near the brain base and ascend to the level of the 
lower surface of the anterior commissure, where they turn abruptly 
cephalad and decussate as they mingle with that commissure. 2d, 
the fibres of the anterior commissure itself, which, as already suggested, 
