1903.] the Retrocalcarine Region of the Cortex Cerebri. 



63 



temporaneously with or perhaps even before the true calcarine sulcus, 

 or, as seems to be the commonest method, in most Apes (certainly in 

 Hylobates) one great fossa develops as the representative of the con- 

 joint calcarine and retrocalcarine sulci. It is certain that the mere 

 chronological order in which sulci develop in different brains is of 

 little value in the identification of their homologies, so that we cannot 

 place such implicit reliance on this method as Cunningham* seems to 

 do when he regards it as the basis on which the accurate comparison 

 of the sulci in the Human and Simian brain must rest. 



In many Apes the area striata, which forms the walls of the retro- 

 calcarine sulcus, may not reach the surface of the cuneus or the 

 lingual gyrus. 



In the brains of certain large Ungulates and Carnivores there is a 

 furrow behind the calcarine (splenial) sulcus, which occupies a position 

 analogous to that of the retrocalcarine in the Primates. In the case 

 of the Camel's brain, I suggested the definite homology of this furrow 

 with the primate retrocalcarine.! In a series of hemispheres of 

 Camelus dromedarius, I have found that the distribution of the area 

 striata closely follows this sulcus. When, as often happens, this 

 retrocalcarine sulcus bends upward to the dorso-caudal angle of the 

 mesial surface, the stria-bearing cortex has a similar peculiar distribu- 

 tion. This places beyond all doubt the question of the identity of the 

 retrocalcarine sulci in the camel and the primates. 



It has been stated by Henschenj; that the stripe of Gennari does 

 not extend on to the lateral aspect of the Human brain. This 

 is true in some, but by no means all, cases. The exact extent of 

 the caudal prolongation of the stria is subject to a wide range 

 of variation. As the area striata is traced backward alongside the 

 sulcus retrocalcarinus, it will be found to expand near the caudal 

 pole^of the hemisphere (fig. 1), but in some cases it does not reach 

 the true lateral aspect. In more than half of the specimens which 

 I have examined, however, it does extend on to the lateral surface 

 (fig. 2) and reaches almost (in some cases quite) as far forward as the 

 sulcus occipitalis lunatus (or " Aftenspalte "). I have chosen to repre- 

 sent as fig. 2 a case in which the sulcus lunatus is placed far back, 

 because this presumably presents the nearest resemblance to the 

 common European type of brain. But it often happens in the 

 Egyptian brain that a large occipital operculum is present, as in 

 the Apes, and in these cases the stria Gennari is prolonged far forward 

 on the lateral aspect into the operculum. 



It has been known for many years that the stria extends into the 

 occipital operculum in the Apes, but I have been unable to find any 



* Op. tit., p. vi. 



f Op. cit., ' Linn. Soc. Trans.,' p. 380, fig. 55. 

 X Op. cit., supra. 



