470 1'IIYSIOLOGICAL SEBI1-X 



In many mammals (such as the Lion) the tension of the growing 

 infracalcarine neopallium is relieved chiefly by the downward exten- 

 sion of the calcarine sulcus toward the posterior rhinal iissure, but 

 also partly by certain irregular and inconstant compensatory sulci 

 behind and parallel to this extension. In the Primates, however, the 

 calcarine sulcus becomes very obliquely placed, not only because the 

 occipital region of the hemisphere becomes caudally extended above 

 the cerebellum, but also because the elongating corpus callosum pushes 

 back, as it were, the pericalcarine neopallium ; and as a result of this 

 obliquity the sulcus cannot be prolonged towards the rhinal lissuiv. 

 as happens in the Carnivora and Ungulata, so that the compensatory 

 sulcus, which is known as the "collateral" sulcus, attains a -really 

 enhanced importance, and fulfils the role of the ventral extension oi 

 the calcarine sulcus. 



A study of the variable collateral sulcus in the brain of Man and 

 the Apes clearly shows its compensatory-calcarine nature. 



Another result of the occipital prolongation of the hemisphere is 

 that the calcarine sulcus becomes widely separated from the intercalary 

 (calloso-marginal) sulcus, to which it is joined in most mammals. 

 The stages in this separation are well shown by comparini:, say. the 

 brain of a typical Carnivore with those of Daubentonia and Ihe Lemurs. 

 As the result of this separation a new set of mechanical conditions 

 prevail in the area between the calcarine and the calloso-marginal 

 sulci; and, to further complicate matters, the arcuate sulcus formed 

 on the dorso-lateral aspect of the hemisphere by the lateral and post- 

 lateral sulci (i. e. the iutraparietal and its ramus occipitalis trans- 

 versus respectively) becomes more and more acutely ncxed us the 

 occipital prolongation occurs, so that in the Cebidae the sharp-pointed 

 apex of the V-shaped sulcus so-formed extends toward this region of 

 the mesial wall, which is, for the reasons just mentioned, already in a 

 state of " unstable equilibrium," so to speak. As the result two sulci 

 (which may, however, be concurrent) are formed: (a) a ventral 

 compensatory-calcarine parallel to the calcarine and the dorsal limb 

 of the postcalcarine sulcus, and (/>) a vertical sulcus cutting into the 

 dorsal edge of the hemisphere. The latter appears to relieve tin- 

 tension of the extending surface in a region which is obviously in- 

 fluenced by the proximity of the " apex" of the intraparietul sulcus. 

 In some cases (e. g. Chrysothriv, D. 554) this sulcus b may be joined 

 to the intraparietal, but in most Apes it is independent of it. Thei\\o 

 sulci a and b usually overlap, and in most cases tin- intervening iryrus 

 becomes submerged so that the two elements appear to form one 

 furrow, which is the " parieto-occipital sulcu*.*' The latter, therefore, 



