318 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



arch, the upper branch receiving the name of fissura occipitalis primus or superior, 

 o\ and the lower that of fissura occipitalis secimdus or inferior, o 2 . Of these the 

 upper or superior occipital fissure, o 1 , has been known as the fissura occipitalis perpen- 

 dicularis, which again has been subdivided into an external portion, the external 

 perpendicular, and a mesial portion, the internal perpendicular or parietooccipital of 

 the human brain. The lower fissure, o 2 , has only been recognized as existing in the 

 human brain by a few of the most recent writers, e.g. by Wernicke, after whom it is 

 sometimes called, — by Wilder, who terms it the exoccipital, and by Schwalbe, who 

 calls it the anterior occipital. These two fissures mark off in all but the highest 

 Simiada? a perfectly smooth or but slightly furrowed occipital lobe, as may be seen in 

 Macaws cynomolgus, PI. XXXVII, fig. 1, which should be compared with figs. 2, 8, 

 9, 12, 19 and 20, PI. XXX VII, and fig. 11, PI. XXXVIII. In Chrysothrix sci- 

 ureus, the upper branch of the arch, o 1 , joins the Sylvian fissure, whilst the lower 

 arch is but imperfectly developed on the lateral surface, although the posterior 

 scroll-like portion is well represented, as can be seen by comparing PI. XXXVIII, 

 figs. 7, 8 and 9. 



The superior occipital fissure, o 1 , in the lower Simians is a perfectly continuous 

 fissure, but as we ascend toward the higher forms there is a tendency to separate this 

 primitively single fissure into two by the development of a small convolution, fig. 13, 

 PI. XXXVIII, 2, from the floor of this fissure. This annectant fold is present in 

 most of the monkeys, but in the greater number it lies entirely concealed within the 

 fissure; it may be seen in the brain of Cebus apella, PI. XXXVIII, fig. 12, marked 

 2 in its concealed position, the fissure o 1 being opened in order to show its relations. 

 It is even better displayed in Afacacus nemestrinus, PI. XXXVII, fig. 5, 2 2 1 . 

 This is the convolution called by Gratiolet the superior external plis de passage. 

 This annectant gyrus is shown as it exists in Man, PI. XL, fig. 1, and PI. XXXVI, 

 figs. 5 and 6. In this way in Man, in the Anthropomorpha, and occasionally in such 

 Simiada* as Ateles, PI. XXXVII, figs. 15 and 16, the parietooccipital fissure is sepa- 

 rated from the lateral surface of the hemisphere. 



The occipital lobe, delimited as above described, is perfectly smooth in such 

 Simiadae as Cercopithecus, PI. XXXVIII, fig. 11, or Afacacus cynomolgus, PI. 

 XXXVII, fig. 20, where the boundaries of the lobe are distinctly outlined by the 

 primary occipital arches. In some species of Macacques it is furrowed, as is shown 

 in PI. XXXVII, figs. 10, 11, 12 and 13, and this fissuration becomes more and more 

 pronounced the nearer we approach toward Man. From this occipital lobe the con- 

 volutions proceed forward to the frontal and temporal lobes. Leaving the considera- 

 tion of the lateral surface of the occipital lobe for the present, we will pass to the 

 consideration of its mesial surface. For this purpose we shall first study its structure 

 as it appears in Chrysothrix, PI. XXXVIII, fig. 8. The mesial extremities of the 

 two fissures, o 1 o 2 , are seen arching, as stated previously around the upper and lower 

 extremities of the transverse calcarine, c a, and in this way is the mesial portion of 

 the occipital lobe completely marked off from the anterior portions except that two 

 small gyri lying one above and one below the fissura calcarina connect it with the 



