316 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



in character than that of the Lemuriadae, but the type stamps them as being more 

 closely related phylogenetically to the higher Simiadae. 



The gradual increase in complexity can be studied in the Simiadse, in the figs, 

 of Plates XXXVII and XXXVIII, where f. 1 or o f 1 is the superior frontal; f 2 or 

 o f 2 , inferior frontal ; f \ inferior vertical frontal or pnecentralis ; f 4 , superior vertical 

 frontal and cc, central fissure. F 1 or OF 1 is the superior frontal ; F 2 or OF 2 , middle 

 frontal ; F 3 or OF 3 , inferior frontal ; AC, anterior central convolution, and o p, oper- 

 culum. From such an examination it will be seen that the vertical portions of the 

 frontal fissures appear first, and the horizontally-running fissure, situated just above 

 the orbital surface, represents merely the horizontal branch of the inferior frontal 

 fissure, which, eventually, joining the vertical portion, becomes as a whole the 

 inferior frontal with its vertical branch. Pansch has considered this portion of the 

 inferior frontal as a distinct fissure in the monkey brain, thus describing three lateral 

 longitudinal frontal fissures dividing the frontal lobe into four frontal convolutions. 

 An attentive study, however, of different monkey brains and comparison with those 

 of the Anthropomorpha will show, I think, that this inferior fissure should be classed 

 with and considered as an after developed portion of his first primary furrow. 



In the Chimpanzee, by the development of bridging convolutions, triradiate and 

 H-shaped zygal sulculi, the frontal surface acquires a complexity which closely relates 

 it not only in general, but also in special characteristics to the human brain. This 

 resemblance can be fully appreciated by comparing fig. 3, PI. XXXVI, which is from 

 the brain of a human foetus toward the end of the ninth month, with fig. 7, or 

 PI. XXXIX, fig. 4, which is a drawing of the left hemisphere of the brain of a 

 Chimpanzee. 



In the parietal region of the Simian brain we find a single well marked fissure, 

 the interparietal, i p, separating two distinct convolutions, the gyrus parietalis 

 superior and inferior, the former of which is directly continuous anteriorly with the 

 posterior central convolution, and, indeed, forms with it a single gyrus ; whilst the 

 latter, extending from the space between the extremities of the Sylvian and interpa- 

 rietal fissures arches upward, backward and then downward, surrounding the posterior 

 extremity of the superior occipitotemporal fissure in the form of an arch, the anterior 

 portion of which has been called the supra-marginal lobule and the posterior the 

 gyrus angularis; In the majority of the Simiadae the interparietal unites with the 

 external portion of the superior occipital fissure, fissura perpendicularis externa, as 

 may be seen by examining figs. 1, 2, 3, 4, 8 and 9, PI. XXXVII, and figs. 1, 2, 3, 4 

 and II, PI. XXXVIII. In Ateles and often in the Anthropomorpha, as we have 

 also found to be the case in Man, these two fissures are separated from each other by 

 a small gyrus which rises from the depths of the superior occipital fissure, the plis de 

 passage stiperieur externe of Gratiolet. This may be seen in Ateles ater, figs. 15 and 

 16, PI. XXXVII, and also in the right hemisphere of the Chimpanzee, fig. 13, PI. 

 XXXVIII. We shall, however, discuss this occipito-parietal region more in detail 

 under the heading of the plis de passage, as it is especially in this region that the 

 most marked changes take place in the human as compared with the Simian brain. 



