x THE FORE-BRAIN 555 



the cortex of the orang do not overlap like those of the lower apes 

 (Macacus, Cerco^ithcrus), hut are separated here and there by 

 intermediate areas which are inexcitable even to strong currents. 

 The sum of the excitahle areas is relatively smaller in the orang 

 than in the lower apes. In tact the first frontal convolution and the 

 upper part of the postcentral convolution were inexcitable ; in the 

 fronto-parietal lohe the whole of the precentral convolution, an area 

 in front of the precentral sulcus, the lower two-thirds of the post- 

 central convolution, and the portion of the marginal gyrus which is 

 continuous with the superior end of the precentral were excitahle. 



Griinbauni and Sherrington obtained somewhat different results 

 from their experiments on sixteen individuals of different species 

 orangs (Opithaecus satyr us), gorillas (Troglodytes gorilla?), and 

 chimpanzees (Troglodytes nigcr and Troglodytes calvus). For 

 stimulating the cortex they preferred the method of unipolar 

 faradisation, hy which the excitable areas can be more precisely 

 differentiated. 



They found in each of the animals examined that the motor 

 areas were present all along the precentral convolution (Fig. 281), 

 and continued into the cortex that dips into the central or 

 Rolandic sulcus, and the other secondary sulci by which this is 

 limited. Probably the excitable area buried in the sulci equals, if 

 it does not exceed, that which is uncovered. The anterior limit 

 of this area is not sharp, and retreats towards the central sulcus 

 when the excitability of the cortex is depressed. The posterior 

 limit, on the contrary, is sharper and more constant, and reaches 

 the floor of the central sulcus along its entire length, with the 

 exception of its upper and lower portions. In none of the animals 

 examined were there excitable areas in the postcentral convolution. 

 Sometimes, and only with strong faradisation, weak and indefinite 

 reactions were evoked, which are not comparable with those 

 obtained from the true motor area. Still it can be seen that the 

 motor effects of faradising the several points of the precentral 

 convolution with weak currents are facilitated by the simultaneous 

 faradisation of the points lying at the same level of the post- 

 central convolution. The student is advised to note this fact, 

 which may solve the contradiction between the results of Beevor 

 and Horsley and those of Grunbaum and Sherrington as regards 

 the excitability of the postcentral convolution. 



The entire surface of the island of Reil is inexcitable even to 

 strong currents. On the mesial surface of the hemispheres the 

 excitable area is small in extent (Fig. 282). It does not reach the 

 sulcus calloso-marginalis. Certain points near this fissure may 

 evoke weak movements of the shoulder, trunk, hand, and finger ; 

 but, according to Grunbaum and Sherriugton, it is uncertain 

 whether these are of the same character as those evoked from the 

 true motor zone. 



