Page:Descent of Man 1875.djvu/219

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Chap. VII.
203
Structure of the Brain.

"And it is especially remarkable that, in the development of the posterior lobes, there is no approximation to the Lemurine, short hemisphered brain, in those monkeys which are commonly supposed to approach this family in other respects, viz., the lower members of the Platyrhine group."

So far as the structure of the adult brain is concerned, then, the very considerable additions to our knowledge, which have been made by the researches of so many investigators, during the past ten years, fully justify the statement which I made in 1863. But it has been said, that, admitting the similarity between the adult brains of man and apes, they are nevertheless, in reality, widely different, because they exhibit fundamental differences in the mode of their development. No one would be more ready than I to admit the force of this argument, if such fundamental differences of development really exist. But I deny that they do exist. On the contrary, there is a fundamental agreement in the development of the brain in men and apes.

Gratiolet originated the statement that there is a fundamental difference in the development of the brains of apes and that of man – consisting in this; that, in the apes, the sulci which first make their appearance are situated on the posterior region of the cerebral hemispheres, while, in the human foetus, the sulci first become visible on the frontal lobes.[1]

This general statement is based upon two observations, the one of a gibbon almost ready to be born, in which the posterior gyri were "well developed," while those of the frontal lobes were "hardly indicated"[2] (l. c. p. 39), and the other of a human fœtus at the 22nd or 23rd week of utero-gestation, in which Gratiolet notes that the insula was uncovered, but that nevertheless "des incisures sèment de lobe antèrieur, une scissure peu profonde indique la séparation du lobe occipital, tres-reduit,

  1. "Chez tous les singes, les plis postérieurs se développent les premiers; les plis antérieurs se développent plus tard, aussi la vertèbre occipitale et la pariétale sont-elles relativement très-grandes chez le fœtus. L’Homme présente une exception remarquable quant a l’époque de l’apparition des plis frontaux, qui sont les premiers indiques; mais le développement général du lobe frontal, envisage seulement par rapport a son volume, suit les mêmes lois que dans les singes"; Gratiolet, 'Mémoire sur les plis cerbères de l’Homme et des Primates,' p. 39, tab. iv, fig. 3.
  2. Gratiolet's words are (l. c. p. 39): "Dans le fœtus dont il s'agit les plis cérébraux postérieurs sont bien développés, tandis que les plis du lobe frontal sont a peine indiques." The figure, however (Pl. iv, fig. 3), shews the fissure of Rolando, and one of the frontal sulci plainly enough. Nevertheless, M. Alix, in his 'Notice sur les travaux anthropologiques de Gratiolet' ('Mém. de la Societe d'Anthropologie de Paris,' 1868, page 32), writes thus: "Gratiolet a eu entre les mains le cerveau d'un fœtus de Gibbon, singe éminemment supérieur, et tellement rapproche de l'orang, que des naturalistes très-compétents l'ont range parmi les anthropoïdes. M. Huxley, par exemple, n'hesite pas sur ce point. Eh bien, c'est sur le cerveau d'un fœtus de Gibbon que Gratiolet a vu les circonvolutions du lobe temporo-sphénoïdal déjà développées lorsqu'il n'existent pas encore de plis sur le lobe frontal. Il était donc bien autorise a dire que, chez l'homme les circonvolutions apparaissent d'α en ω, tandis que chez les singes elles se développent d'α en ω."