Visuo-Sensory aiirf Visuo~Psyclu<- Areas [CHAP. 



gyrus ciinei, joining the cuneus to the post-limbic convolution, has come to the surface 

 and completely separated the parieto-occipital from the calcarine fissure. I have stated already 

 that in the human subject this annectant lies at a deep level and is seen only when the 

 lips of the fissure are thrown widely apart, also I have pointed out that the calcarine 

 type of cortex is to be found on the lower or calcarine, but not on the upper or parieto- 

 occipital half of this annectant. Now in some cases in the human being 3'9 per cent. 

 according to Cunningham the gyrus cunei comes to the surface in anthropoid fashion, and 

 it is likely that in these instances the calcarine type of cortex is drawn up to a corre- 

 sponding degree and that it may cover some of the upper wall of the "stem" of the fissure, 

 as it does in the case of the ape. 



Before considering the relation of this type of cortex to the fissura calcarina posterior, I 

 have to mention that, in the memoir already quoted from, Cunningham maintained that this 

 division of the fissure, fundamentally distinct from the anterior division in the human brain, is 

 not represented in the ape. His grounds for this statement are as follows, " throughout its 

 whole length the calcarine fissure in both the chimpanzee and the orang presents very nearly 

 the same depth. Its walls are smooth, and there is not a vestige of a deep annectant gyrus 

 to be seen crossing its bottom at any point to connect its opposite banks with each other. 

 Further, when we study the relation which this fissure presents to the calcar avis, we observe 

 that the entire length of this ventricular eminence corresponds with that portion of the sulcus 

 which lies opposite to it." Hence he believes that the entire length of the precursor of 

 the calcarine fissure in the early human foetus is the equivalent of the calcarine fissure 

 of the ape, that m the ape the original fissure persists in its entirety, and that the posterior 

 portion does not become obliterated, to be replaced at a later date by the secondary posterior 

 calcarine fissure, as happens in man. 



I shall have occasion to return to this question of the development of the calcarine 

 fissure, I mention it here before I describe the relation which the calcarine type of cortex 

 bears to it, only to indicate that a certain element of dubiety surrounds the morphological 

 representation of the fissura calcarina posterior. 



Now we find this type of cortex clothing the floor and both the cuneal and the lingual 

 walls of this portion of the sulcal complex, also we see it spreading for a distance of 3 5 mm. 

 on to the exposed surface of the cuneus above and of the lingual lobule below. Further, as 

 the pole of the hemisphere is approached the area undergoes a marked expansion. In all 

 these respects we have a reproduction of the human arrangement, and I may say that in 

 the second chimpanzee's brain which I examined the arrangement is extremely like that seen 

 in the human being. 



But when we reach the margin of the hemisphere the resemblance ceases, for instead 

 of curving round the hinder margin of the hemisphere for a distance of about 1 cm. and 

 then ceasing, as it does in the human brain, the area is carried on for at least 4 cm. in 

 the horizontal plane, terminating only a short distance behind the lower extremity of the 

 " Affenspalte " ; and the field which it covers is by no means a narrow strip, for, viewed 

 from behind, it must be described as occupying the major part of the postero-lateral surface 

 of the occipital lobe. 



And next the question arises, how far can the disagreement in territorial distribution 

 of this type of cortex in the human and anthropoid brains be made to coincide with the 

 anatomical discrepancies which apparently exist ? 



