439 



Much as writers on this subject differ as to the exact limits of 

 the cortical "visual centre" I think that it is now generally recognised 

 that the area striata forms the essential part of the receptive appa- 

 ratus for visual impressions. 



The area striata is situated partly on the mesial and partly on 

 the lateral aspect of the hemisphere and the exact proportion of the 

 pars mesialis to the pars lateralis varies in different Apes and in Man. 

 The great distinctive feature of the area striata (when contrasted with 

 most other regions of the neopallium) is the fact that it undergoes 

 little if any increase in size in the series consisting of the old world 

 Apes and Man: and, as the neopallial region immediately in front of 

 the lateral part of the area striata undergoes a progressive expansion, 

 the stria -bearing cortex gradually becomes pushed on to the mesial 

 surface. In many human brains the area striata becomes wholly re- 

 legated to the mesial surface : but in other cases more than a quarter 

 of it may still retain its primitive situation on the lateral surface. 

 In the smallest Cercopithecidae, in which the great parietal expansion 

 has not yet become specially pronounced, the pars lateralis may in- 

 clude as much as one half of the stria-bearing cortex. 



The area striata begins (on the mesial surface) a short distance 

 behind and on a slightly lower level than the splenium of the corpus 

 callosum : at first it is a narrow band but it rapidly broadens (i. e. its 

 depth or vertical extent increases) as it is traced backward; and on 

 the occipital pole it occupies the whole depth of the mesial surface. 

 In such Apes as the Cercopithecidae it occupies the whole depth of 

 the lateral surface of the hemisphere from the dorso-mesial edge above 

 to the ventro-lateral edge below: and it extends forward as far as the 

 free margin of the occipital operculum in most cases. Thus the area 

 striata is separated from the floor of the Sulcus lunatus by the distance 

 between the latter and the free edge of the operculum. In those Apes, 

 such as many examples of Cebus, in which there is no occipital oper- 

 culum the stria Gennari extends as far as the caudal lip of the verti- 

 cally incised sulcus lunatus. The occipital operculum is, in fact, a kind 

 of mushrown - like outgrowth formed by the forward expansion of the 

 area striata — the superficial layer of the mushroom. It is therefore 

 not certain whether we are justified in speaking of the sulcus lunatus 

 ("Atfenspalte") as being strictly homologous in the brains of various 

 Apes, for, although the superficial layer of its caudal lip, whether the 

 latter be operculated or not, is formed of area striata the distance 

 between the floor of the sulcus and the edge of the operculum (or of 

 the area striata) is variable. 



