MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 329 



In Ateles ater we find a brain in which the primitive arches are somewhat 

 modified and assume a tortuous appearance, the relations to the original morpho- 

 logical type can, however, be readily ascertained by comparing, Plate XXXVII, fig. 

 16, with figs. 15 and 18. The primary occipital arch has been separated in its upper 

 branch, the fissura occipitalis superior, into a mesial, O 1 , fig. 17, Plate XXXVII, and 

 a lateral branch, O 1 , figs. 15 and 18, forming an internal and external (so called) 

 perpendicular fissure, by the development of the superior external pli de passage, 2. 

 This development corresponds with a similar condition of affairs in the Anthropo- 

 morpha and in the human brain. Otherwise, the remaining parts are as already 

 described, except, as we have remarked, the fissures assume a more tortuous aspect. 

 Thus, O 1 is the superior occipital fissure ; O 2 is the inferior occipital fissure ; the 

 secondary arch is imperfect anteriorly, but its two elements, O 1 ' and O 2 ', are easily 

 distinguished separating three occipital gyri, S. Oc, M. Oc. and I. Oc. 



The appearance of the portion of the cerebral surface in Ateles in front of the 

 occipital lobe is markedly modified by the backward extension of the fissure of 

 Sylvius up to the border of the hemisphere, relating it in this respect to the 

 arrangement as found in Chrysothrix sciureus, Plate XXXVIII, fig. 6, where the 

 Sylvian becomes continuous with the fissura occipitalis superior as it does also in 

 the right hemisphere of this specimen of brain of Ateles, figs. 15 and 16, Plate 

 XXXVII. 



In the Chimpanzee, as a representative of the Anthropomorpha, the plan of 

 occipital morphology above described is very evident. See Plate XXXVIII, fig. 16. 

 In the left hemisphere, fig. 13, the superior occipital fissure, O 1 , is unbridged super- 

 ficially, but the pli de passage sup'erieur externe, 2, really exists in this specimen, 

 being only imperfectly concealed. The inferior occipital, O 2 , is well marked, as is 

 also the secondary occipital arch, O 1 ' and O 2 ', separating the occipital lobe into three 

 gyri, S. Oc, M. Oc. and I. Oc. In the right hemisphere the convolution, 2, 

 reaches the surface, dividing the fissura occipitalis into a mesial and a lateral por- 

 tion. (See Plate XXXVIII, fig. 16.) 



In Man the complexity of this lobe reaches its highest development, and no 

 two anatomists view it entirely in the same light. It also varies greatly in regard 

 to its complexity in different individuals. In some it will be found comparatively 

 simple, whilst in others, from the excessive development of secondary and tertiary 

 sulculi and rami, it assumes at first sight a puzzling appearance. I have always 

 found it, as a rule, simpler in the Negro than in the White. In fact it was mainly 

 through the attentive study of Negro brains that I was able to determine that the 

 arrangement of the occipital lobe in Man is of the same general type as we have 

 found it in the Simiadaa ; only rendered less distinct by tortuosity, and the devel- 

 opment of sulculi. I cannot agree with the conclusion of Ecker and others that 

 the arrangement of the fissures in Man is very different from that in the monkeys. 

 I believe that studied from the morphological standpoint advocated in this paper 

 they will be found to be based upon a similar type. 



