Herrick, Cortical Motor Centres. 95 



"Perhaps more important in its bearing on the present sub- 

 ject is a set of experiments upon the opossum, of which, unfor- 

 tunately, no very exact data have been preserved. The con- 

 figuration of the hemispheres as well as the details of structure 

 resemble very closely those of rodents. Moreover, such hints 

 as we have of the development of the Rodentia indicate a com- 

 mon origin for the two groups and comparatively slight subse- 

 quent differentiation. It, then, would not be surprising if a 

 considerable similarity of distribution in the cortical elements 

 should be proven to exist. On the other hand, the existence 

 of an apparent homologue of the crucial sulcus near the front 

 of the cerebrum would lead one to expect the aggregation of 

 the motor elements near this sulcus. The experiments in this 

 case were made with a Grenet cell and DuBois-Reymond coil, 

 with the secondary coil at about 8 cm. the current being applied 

 by a pair of platinum electrodes separated by about 3 mm. 

 Stimulation of the region about the crucial sulcus (so-called) 

 resulted in movements of the anterior extremity, but the diffi- 

 culty in controlling the flow of blood interfered with close anal- 

 ysis. The area on either side of the median fissure responded 

 with various poorly localized contractions of the trunk. About 

 6-8 mm. posterior to the crucial sulcus and 4-5 mm. from the 

 medial line is an ill-defined area governing the hind leg. These 

 motor reactions were, in the main, crossed as usual, but in sev- 

 eral instances similar motions of the muscles of both sides re- 

 sulted when superficial irradiation appeared to be excluded. 

 The areas thus roughly mapped in the opossum coincide in gen- 

 eral with those of the ground hog and we are forced to conclude 

 that the crucial sulcus of the opossum is not strictly homolo- 

 gous with the fissure so named in carnivora. " 



Cunningham locates the motor centre for the fore limb 

 along the caudal boundary of the so-called crucial sulcus and 

 doubtfully reports an area for movements of the ear in the lat- 

 eral region, pretty well caudad. He states that there is no def- 

 inite centre for the hind leg or, at least, that if the animal is 

 thoroughly under the influence of the narcotic no motion can 

 be evoked by electrical stimulation. Ziehen, on the other hand, 



