ix] Intermediate Prcn-ntral Ari 215 



judgments on cell size and cell lamination are thereby rendered more difficult. I have found 

 my sections stained for nerve fibres preferable for general working purposes, and in these the 

 points which have served best as guides are (1) a wealth of fibres in the radiary zone strictly 

 intermediate between that seen in the " precentral " area behind and that in the area next in 

 front, ('!} the presence in the radiary zone of fibres of the large evenly-medullated type, and 

 particularly of such fibres placed in a horizontal position, (3) a zonal layer also of intermediate 

 representation and containing some fibres of larger calibre than any seen in more frontally-placed 

 areas, and (4) a general depth of cortex, not less than that in the "precentral" field, but greater 

 than the same in the prefrontal area. 



But it is the distribution more than the histological structure of this area which is so 

 interesting. 



Starting with its extent on the mesial surface of the hemisphere. Here the human plan 

 is closely followed : behind, it is of course in contact with the " precentral " area ; below, the 

 calloso-marginal sulcus forms a definite boundary, for there is no mistaking the difference 

 In -r\veen this cortex and that of the subjacent calloso-marginal or fornicate gyrus ; anteriorly 

 I can find no named sulcus or part which can be used for purposes of delimitation, the margin 

 curves upwards and crosses the border of the hemisphere at a point about 2 cm. in front 

 of the " precentral " area and about 3'5 cm. in advance of the upper extremity of the fissure 

 of Rolando, at least that is what happens in the case of the chimpanzee, in the orang the extent 

 is somewhat greater. 



On the convexity, the great breadth of the area, as seen on the mesial surface, is preserved 

 only for a short distance; at first the anterior border lies about 15 mm. in front of the sulcus 

 precentralis superior, a sulcus easily defined in all three specimens submitted to examination, 

 but instead of proceeding downwards parallel to the fissure of Rolando, the margin crosses 

 the upper frontal tiers of convolutions more or less vertically, and the next landmark we can 

 make use of is the upper extremity of the sulcus arcuatus, or as I shall speak of it hereafter, 

 the sulcus precentralis inferior l . Now to this furrow the area evidently bears the same relation 

 as in the human being, only in all these specimens the intermediate characters seem to 

 cross the fissure and spread on to its anterior lip, while in man they remain behind. In this 

 position, opposite to the so-called horizontal ram us of the inferior precentral furrow, the area 

 is seen at its narrowest. 



My description of the remainder of the area must be prefaced with some explanatory anatomical 

 observations, for the disposition of suk-i and gyri in the lower frontal region is distinctly confusing to those 

 unfamiliar with the anthropoid brain, and as views on their hoinologies are discordant it is essential that 

 my description shall be free from ambiguity. Now in addition to the sulcus arcuatus, to which reference 

 has already been made, there are four furrows to which I invite attention : the inferior transverse sulcus 

 of the fissure of Rolando, the sulcus rectus, the anterior Sylvian and the fronto-orbital fissures. 



The inferior transverse sulcus of the fissure of Rolando is soon dismissed, for it holds an almost identical 

 position with the human element having the same name. It is present in all my specimens, in one chimpanzee's 

 hemisphere (right side) it is isolated, but in the other and also in the orang's it effects a shallow junction 

 with the lower extremity of the sulcus precentralis inferior, the. union being marked by a submerged 

 annectant gyrus. 



1 Differences of opinion have been expressed ou the homology between the suleus arcuatus of the ape and the 

 human sulcus precentralis inferior, but it is satisfactory to know that Eberstaller, Waldeyer, Cunningham, and 

 Elliot Smith are now unanimous in supporting the correspondence. 



