74 



Motor Localisation in the Brain of the Gibbon. 



It will be seen on comparing these figures with Campbell's diagrams of 

 the brains of the Orang and Chimpanzee, that the distribution of the Betz 

 cells is very similar in all three cases. The G-ibbon presents perhaps a 

 slightly closer resemblance to the Orang in this respect than to the 

 Chimpanzee. 



It is the distribution of the intermediate precentral area which forms the 

 most characteristic feature of the Gibbon's brain. The great forward 

 extension of this area distinguishes it in a very striking way from the 

 Orang and Chimpanzee, on the one hand, and Cercopithecus and the Baboon 

 on the other. This extension is most marked in the region which may 

 be described as the middle frontal convolution, namely, that portion of the 

 lateral surface which lies between the sulcus precentralis superior (prs.) 

 above, and the sulcus rectus (red.) below. The area occupied by the 

 granular frontal cortex (Campbell's frontal cortex and Brodmann's type 9) 

 becomes in this way very much restricted, and above the sulcus rectus it 

 occupies only the very small space in the neighbourhood of the frontal 

 pole indicated in fig. 3 by small circles. Below that fissure the layer of 

 granules or stellate cells is well developed in nearly the whole region 

 lying in front of the fronto-orbital sulcus (/<?.). 



Probably as a result of the great development of the intermediate 

 precentral area the sulcus arcuatus, the upper limit of which in Cercopithecus 

 and the Baboon arches round the posterior end of the sulcus rectus, and 

 lies just within or actually forms a boundary to this area, has been pushed 

 downwards to such an extent that it has become continuous with that 

 fissure. This condition can be recognised most clearly in the left hemisphere, 

 where the sulcus rectus has posteriorly a well developed downwardly directed 

 limb, which is clearly the homologue of the lower portion of the sulcus 

 arcuatus ; in the right hemisphere it is very difficult to recognise the latter 

 at all. 



Another point worthy of attention is that in the cortex of the posterior 

 part of the middle frontal gyrus the large ceils of the ganglionic layer, or 

 inner layer of large pyramids, are somewhat larger than in the region lying 

 above the anterior end of the sulcus precentralis superior, or below the 

 sulcus rectus, but are not nearly so large as those which have previously 

 been referred to as unquestionable giant pyramids. 



