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 oj^ossum 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 simj^licity of structure. The smooth, depressed and elon- 

 gate hemispheres, protruding and exposed olfactory lobes, and partially 

 exp jsed 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 relation to the hippocampus and the 

 great size of the ventricle.s. In spite of the considerable size of the 

 cere'.)^llum, it does not entirely cover the fourth ventricle. The sev- 

 eral spinal nerves have a degree of independence greatly facilitating 

 their study. 



T.ie 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 DideLpIiys the callosum is practically absent and the fibres serv- 

 ing the function of the corpus callosum in higher mammals are associ- 

 ated with the anterior commissure. The structure of this region in 

 the op;;;. in is of great interest. The great mass of transverse fibres 

 cDns^itLitiag the commissural system of the fore-brain consists of four 

 elements, viz : 



I St. Fibres from the olfactory tract which cross from the pyriform 

 lobj t> 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 comniissure. 2d, 

 the fibres of the anterior commissure itself, which, as already suggested. 



