236 Condition in the Anthropoid Ape [CHAP. 



DISTRIBUTION OF THE "FRONTAL" AND "PREFRONTAL" AREAS. (Plate XXII.) 



The distribution of these areas was roughly indicated at the beginning of this chapter, 

 and now reference to the diagrams on Plate XXII will assist the interpretation of the 

 following details. 



A. " Frontal " Area. 



First, taking the part marked " frontal " : on the mesial surface of the hemisphere, it 

 occupies a small portion of the marginal gyms, lying anterior to that covered by the " inter- 

 mediate precentral " type and above the level of the callosal genii. 



On the lateral surface it is wide in extent. The posterior boundary need not concern 

 us, because it has already been described in connection with the " intermediate precentral " 

 area. Confining our attention therefore to the anterior limit, we see that it crosses the upper 

 margin of the hemisphere about 4 cm. above the fronto-orbital border, that is, close to the 

 point where the sulcus frontalis superior vel primus usually ends. From this point it pursues 

 a sinuous course downwards, first crossing the sulcus frontalis medius and then making for 

 the fronto-orbital margin of the hemisphere near the most lateral part of the sulcus frontalis 

 marginalis of Wernicke. From here it passes on to the orbital surface, where the external 

 sagittal branch of the orbital sulcus first forms part of the boundary, but as the sulcus 

 orbitalis transversus is approached, it crosses the first named fissure and then arching round 

 in front of the latter it ends close up to the root of the sulcus olfactorius. 



Therefore on the lateral surface the area covers roughly-speaking the anterior half of 

 the superior frontal gyrus, nearly the whole of the middle frontal gyrus (the most anterior 

 and most posterior portions being excluded), and that portion of the inferior frontal gyrus 

 lying between the anterior vertical portion of the sulcus frontalis inferior and the lateral 

 extremity of the sulcus frontalis marginalis of Wernicke. 



On the orbital surface it covers a small area lying lateral to the external sagittal sulcus 

 and a thin strip anterior to the sulcus orbitalis transversus. 



B. " Prefrontal " Area. 



The remainder of the frontal lobe, excluding that covered by the "limbic" type, belongs 

 to the " prefrontal " area. 



On the mesial surface of the hemisphere it comprises that portion of the marginal gyrus 

 lying anterior and ventral to the callosal genu, or to the prelimbic division of the calloso- 

 marginal fissure. 



On the lateral or frontal surface, the field is small and only takes in the most anterior 

 portion of the middle frontal gyrus. 



On the orbital surface it is extensive, it virtually covers all save that behind the sulcus 

 orbitalis transversus and that without the external sagittal sulcus. 



FRONTAL AND PREFRONTAL AREAS IN THE ANTHROPOID APE. (Plate XXII.) 



Having given particular care to the examination of this part of the frontal lobe in the 

 anthropoid ape, I am able to say without any hesitation that it is histologically divisible 

 in the same way as is the corresponding field in the human brain. Also, I may mention 



