450 



the relations of the doubtful sulci to it there can then be no difficulty 

 in identifying with absolute certainty honaologous sulci in ditferent 

 brains. If in addition we describe the sulci in reference to their re- 

 lation to the area striata, we can thereby avoid the confusion of the 

 common nomenclature. 



Perhaps the most striking result of this investigation is the de- 

 monstration of the fact that the sulci called "calcarine" in most human 

 and all Simian l)rains respectively are not strictly homologous. 

 The so-called "calcarine fissure" of the Apes is a complete involution 

 of the whole mesial part of the area striata, a fossa striata occipitalis : 

 whereas the similarly named furrow in the human brain consists in most 

 cases of anterior and posterior parts which are genetically distinct — 

 the anterior part being the anterior limiting sulcus of the mesial area 

 striata, sulcus praestriatus, and the posterior part a mere depression 

 in (not a complete infolding of) the mesial area striata, sulcus intra- 

 striatus. The cortical areas in the Apes' brain, which are homologous 

 to the ventral part of the human cuneus and the dorsal part of the 

 lingual lobule, are buried in the fossa striata (so-called "calcarine 

 fissure"), so that any comparisons of the cuneus of the human brain 

 with the superficial "cuneus" of the Apes will be very misleading if 

 the fact be ignored that the former is partly striate and partly non- 

 striate cortex whereas the latter is wholly non-striate. 



No better illustration of the limitations to the homologising of 

 cerebral sulci could be atiorded than that which I have just been dis- 

 cussing. Every other writer (with the exception of Cunningham) has 

 hitherto assumed that the sulcus called "calcarine" in the Apes is a 

 strict homologue of that similarly labelled in the human brain; but, 

 as I have just shown, this is true only in a very limited sense. In 

 fact, the common human condition is more nearly realised in the 

 Lemurs and many other mammals than in the Apes. 



In most mammals other than the Primates the ditferent rates of 

 growth of the cortical receptive centre for visual impulses (area striata 

 occipitalis) and the gyrus fornicatus calls into being the sulcus prae- 

 striatus (splenialis, calcarinus proprius), which separates the two re- 

 gions. In many mammals (such as the large Carnivora, Ungulata and 

 Cetacea) the expansion of the area striata produces furrows within it 

 (intrastriate sulci). The direction of these furrows is determined by 

 purely mechanical conditions: e. g. in most mammals the caudal ends 

 of the hemispheres are blunt and the postsplenial (intrastriate) sulcus 

 pursues a course midway between the praestriate (splenial) sulcus and 

 the caudal margin of the hemisphere to both of which it is parallel. 



