MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 289 



rior part of this fissure becomes separated from the anterior by a small convolution, 

 g. f., fig. 4, Plate XXXVI. This is the convolution called by Huxley and Ecker 

 the gyrus dentatus, the corps godronne of Gratiolet and others. It has its origin 

 back of the splenium corporis callosi and connects the gyrus fornicatus, g. f., with 

 the hippocampal. 



It has been stated that the presence of this convolution and the separation in 

 consequence of its development of the calcarine, c.a., from the hippocampal, Hp, is 

 a characteristic of the human as distinguished from the ape brain. On this point 

 Ecker remarks as follows : " there is hardly a point in which the brain of most 

 apes is so essentially distinguished from that of Man as in regard to the just 

 mentioned fissura hippocampi. In Man, as we have seen, the gyrus fornicatus 

 passes without interruption into the gyrus hippocampi, and thus the fissura cal- 

 carina and fissura hippocampi are separated from each other by this continuous 

 series of convolutions. In the apes, on the contrary, so far as known, with the 

 exceptions of Ateles and Hylobates, the two just named fissures pass immediately 

 into each other and thus in them the gyrus fornicatus and gyrus hippocampi 

 are separated from each other. Hence Gratiolet who derived his whole termi- 

 nology from the ape's brain, described the two fissures which we separate as fissura 

 calcarina and fissura hippocampi as one, the scissure des JiippocampesT 



With this I can by no means agree. I have examined in respect to this point 

 the brains of eighteen monkeys of different genera and species, and in every 

 instance this convolution was plainly developed. It may be seen marked x in 

 Macacus nemestriims, fig. 5, Plate XXXVII, completely separating the deeper por- 

 tions of the calcarine, c.a., from the hippocampal, Hp. It appears absent in the 

 monkeys because the upper edge of the convolution lying below the calcarine 

 fissure shelves over and conceals it. If this be lifted, however, it will be found 

 extending across the calcarine fissure and separating it from the hippocampal. 

 The nature of these relations is well seen by comparing this gyrus dentatus as it 

 exists in the chimpanzee, g.d., fig. 3, Plate XXXIX, with its condition as found in 

 the brain of a negro, fig. 2, Plate XLI, x. Even in so low a simian as the American 

 monkey Chrysothrix, Plate XXXVIII, fig. 8, I found it distinctly developed, 

 although sunk deeper into the depths of the fissure than in the higher monkeys. 

 The human foetus at the middle of the fifth month presents in this respect a 

 condition almost identical with that found in Chrysothrix. The calcarine joins 

 the hippocampal at the upper part of its depth, but if the sides of the fissure 

 be pushed a little apart, the convolution may be seen stretching across the calca- 

 rine and separating it completely at its lowest part from the hippocampal. In 

 the brain of a foetus at the middle of the sixth month this convolution is devel- 

 oped well up to the surface and presents the same relations as found in such 

 monkeys as Macacus, Cercopithicus, etc. In the brain of a foetus of eight months 

 it was just perceptible superficially, Plate XXXVI, fig. 2, g. f., in this respect 

 representing the condition as found in Ateles, Plate XXXVII, fig. 17, and Hylo- 

 bates. As further evidence that there is nothing peculiar in this convolution 



