MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 325 



o' o 2 ) is becoming greatly modified by being pushed backward and compressed 

 into a much smaller relative space. As a result of these conditions the fissures 

 have a bridged and highly tortuous appearance, assuming, as it were, a rudimentary 

 condition as compared with homologous parts in the monkey and ape brain, the 

 lobe on the whole having the appearance of being crowded into insufficient space. 

 On this point Wilder, in a foot note, remarks. l may these characteristics be cor- 

 related with the fact that the occipital lobe is almost if not quite confined to the 

 Primates, and is, so to speak, a 'new thing' in Nature. 1 The superficial smoothness 

 of the monkey's lobe is only apparent, the poma (' operculum occipitale') really 

 involving a very peculiar and considerable complication which I hope to discuss in 

 a future paper." 



INFERIOR BRANCH, O 2 , OF THE PRIMARY OCCIPITAL, ARCH. 



We have already discussed the presence and arrangement of this fissure in 

 considering the boundaries of the occipital lobe in the Simiadse. It can be well 

 seen as represented in them in figs. 1, 2, 5, 6, 7, 8, 9, 10, 14, 17 and 18, Plate 

 XXXVII; 1, 2, 4, 5, 8, 11,12 and 16, Plate XXXVIII; in figs. 3 and 4, Plate 

 XXXIX, as found in the Anthropomorpha, represented by the Chimpanzee. I 

 have termed it the fissura occipitalis inferior or secunda. It has also been known 

 as Wernicke's fissure, exoccipital of Wilder, anterior occipital of Schwalbe and 

 sulcus occipitalis longitudinalis inferior of Ecker. 



This fissure, together with the fissura-occipitalis superior or prima, O 1 , form- 

 ing the superior branch of the occipital arch, are the only two fissures that I con- 

 sider of primary significance as respects the lateral surface of the occipital lobe, 

 and together they cut off and separate it from the rest of the hemisphere forming 

 the morphological boundaries of this lobe. I will endeavor, further on, by a com- 

 parison of the fissures and convolutions of this lobe in Man and the higher 

 Simiadge, to give my reasons for this opinion. This inferior occipital fissure is 

 always found well marked in the human brain, as may be seen by examining the 

 various figures given in the plates. These two fissures rightly identified and 

 their importance recognized, the complexity of the human occipital lobe in a great 

 measure disappears. From this occipital lobe, 0, contained within the boundaries 

 of these two fissures, O 1 and O 2 , pass forward both in the monkeys and in Man the 

 fissures and convolutions to the extremities of the occipito-temporal and occipito- 

 frontal or fronto-parietal lobes ; whilst posteriorly, the occipital lobe extends around 

 upon the mesial surface and becomes continuous beneath the extremities of the 

 fissures, O 1 and O 2 , with the mesial occipito-frontal and mesial occipito-temporal 

 convolutions, as may be seen in fig. 2. Plates XLII and XL1V (Man), fig. 3, Plate 

 XXXIX (Chimpanzee), and fig. 5, Plate XXXVII (Macacus), etc. 



On the lateral surface of the occipital lobe the separation by the fissures, Oi 

 and O 2 , is also not quite complete, the apex, x, fig. 4, Plate XXXIV, running for- 

 ward and upward to join the angular gyrus, P 2 1 , or posterior portion of the gyrus 



