214 Condition in the Anthropoid Ape [CHAP. 



frontal direction it leaves this fissure to pass obliquely upwards and forwards to the margin of 

 the hemisphere. Below the " precentral " area (still on the same surface) it merges with the 

 " intermediate postcentral " field. 



Along the upper surface of the hemisphere the area is again extensive, reaching well 

 forwards on to the superior frontal gyrus, and it is interesting to notice that it resembles the 

 " precentral " field in attaining its greatest breadth in this its uppermost division 



Below the level of the superior frontal gyrus the zone rapidly narrows and becomes 

 confined to the anterior half of the ascending frontal or anterior central gyrus and to a small 

 portion of the base of the middle frontal gyrus. 



The behaviour of the area in its further course downwards is distinctly peculiar ; instead 

 of ceasing as the " precentral " cortex does towards the lower end of the fissure of Rolando, it 

 sweeps forwards across the deep sulcus precentralis inferior, then after covering most of the 

 surface of the inferior frontal gyrus, including some but not all of the pars basilaris, and the 

 whole of the pars triangularis l , it turns inwards and creeping along the orbital operctilum is 

 not arrested until the sulcus transversus orbitalis is reached. 



The sulcus transversus orbitalis and the calloso-marginal fissures, at the upper and lower 

 extremities of the area, respectively, seem to be fixed limits, but excepting these, the boundaries 

 of the field are not determined or even directly influenced by any fissures, and particularly 

 not by the system of precentral sulci, save the upper part of the inferior precentral element. 

 It will be remembered that the anterior boundary of the " precentral " area proper was similarly 

 independent. 



The extent of this area on the lateral and mesial surfaces may be seen in Plates I and XXII, 

 and its distribution on the orbital surface is specially indicated in the latter Plate. To prevent 

 misconception I would add that the area never extends on to the insula proper. 



INTERMEDIATE PRECENTRAL AREA IN THE ANTHROPOID APE. (Plates II and XXII.) 



As the " intermediate precentral " area forms an integral part of the frontal lobe, and as 

 the condition of the frontal lobe is one of our best criteria in estimating comparative grades 

 of cerebral organisation, the study of this area in the anthropoid brain is full of interest. 

 Being but one step down in the phylogenetic scale and having, as we already know, a " pre- 

 central " area remarkably akin to that in the human being, it would be strange if the 

 development of the " intermediate " area in these animals exhibited a pronounced departure 

 from the human state : and in the following lines I hope to show that, so far as the representation 

 of the field is concerned, the resemblance is maintained, and that thereby the name anthropoid 

 applied to these beings receives further justification ; also, in describing the area I shall give 

 more details than I have in the case of most other areas in the ape. 



First, with regard to structure : in this respect it bears a pronounced resemblance to 

 the human cortex, and the same cell and fibre characters can be made use of in 

 determining its boundaries. But as all these features are reproduced on a small scale, 



1 The tendency seems to be for the "intermediate" type to cease along the lip of the Sylvian fissure; in this 

 way the exposed part of the frontal operculum comes within the area, while the covered part takes on the characters 

 of the insular cortex. In some cases the insular type appears to creep up and cover an appreciable extent of the 

 exposed part of the pars triangularis. It will be noticed that in the anthropoid brain much more insular cortex lies 

 exposed than in man. 



