ii8 Journal of Comparative Neurology. 



instance, belongs partly to the first, partly to the second, and 

 partly to the third frontal gyrus, and was examined in connection 

 with these three divisions. But the entire orbital surface shows 

 such uniformity of structure that it requires but a single set of 

 measurements, and is therefore treated in this paper as a single 

 region. The first frontal gyrus, on the other hand, which in the 

 customary division of the cortex is treated as one region, shows 

 such variations in structure that it is necessary to break it up 

 into four separate parts ; first the orbital surface which is in- 

 cluded in the orbital region ; second the anterior third of the 

 lateral surface together with the adjoining portion of the mesial 

 surface, which form an entire region by themselves ; third the 

 posterior two-thirds of the lateral surface which forms another 

 single region ; and fourth the portion of the mesial surface bor- 

 dering on the sulcus calloso-marginalis, which is included with 

 the gyrus fornicatus in still another region. 



Dividing the entire cortex after this fashion, on the basis 

 of histological differences, sixteen structural regions have been 

 distinguished, which have been numbered as follows : 



Region i . The orbital surface of the frontal lobe. 



Region 2. The anterior third of the lateral surface of the 

 first frontal gyrus plus the adjoining portion of the mesial 

 surface. 



Region 3. The entire lateral surface of the second fron- 

 tal gyrus. 



Region 4. The portion of the lateral surface of the third 

 frontal gyrus anterior to the ascending branch of the Sylvian 

 fissure. 



Region 5. The posterior two-thirds of the lateral surface 

 of the first frontal gyrus. 



Region 6. The portion of the third frontal gyrus poster- 

 ior to the ascending branch of the Sylvian fissure. 



Region 7. The anterior central gyrus plus the upper sixth 

 of posterior central. 



Region 8. The lower five sixths of the posterior central 

 gyrus. 



Region 9. The superior and inferior parietal gyri. 



