328 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



troisihne pli de passage externe. The secondary arch runs parallel with the first, 

 and as this arch becomes successively more and more tortuous, the secondary also 

 increases in complexity. On the right side of fig. 20 can be seen the primitive rela- 

 tions of the mesial surface of the occipito-frontal and temporal lobes to the fissures 

 of the occipital lobe, as explained in our previous discussion of this portion of the 

 hemisphere. The morphological plan of the occipital lobe is here expressed in its 

 simplest condition, and it now remains for us to show the various modifications 

 that take place as we advance from the lowest to the highest Primate brains. 



In the Lemuridse the occipital lobe is entirely smooth. The superior occipital 

 fissure, O 1 , is, however, present, as may be seen in Lemur nigrifrons, Plate 

 XXXIV, figs. 16 and 17, also in Propithecus, Plate XXXIV, figs. 14 and 23, in 

 Avahis, Plate XXXIV, figs. 18, 19, and 20, and in Indris, Plate XXXIV, figs. 6, 

 7 and 8. In Cheiromys there is no sign of either arch, the occipital lobe being 

 hardly defined. 



In the Marmosets, the whole lateral surface of the hemisphere is smooth, as 

 in Hapale and Midas, Plate XXXIV, figs. 11, 12, and 13, there being no signs 

 of the appearance of the occipital fissures. In all the Simiadge the primary 

 arch is well developed, and cuts off at first a smooth occipital lobe which gradually 

 becomes furrowed by the formation of a secondary arch, as shown in Macacus 

 nemestrinus, Plate XXXVII, fig. 10. In Macacus cynomolgus, figure 20, the 

 occipital lobes are entirely smooth, and the relations of the secondary arch to the 

 primary is well shown by comparing this brain with that of Macacus nemestrinus. 

 Sometimes the whole of the secondary arch is not present, and it appears broken 

 up into two or more parts as in another specimen of M. nemestrinus shown in Plate 

 XXXVII, fig. 11, where the lower branch is present, but the posterior middle por- 

 tion is absent with the exception of the most posterior scroll-like portion, which 

 is present. Plate XXXVII, fig. 13, represents an unknown species of Macaque; 

 the same relations obtain as in fig. 11, with the exception that the secondary arch 

 is deeper. The posterior portion of the imperfect upper branch is seen on the left 

 hemisphere. It will also be noticed that the inferior branch, O 2 , of the primary 

 arch, the fissura occipitalis inferior, is assuming a slightly undulating course. 



Plate XXXVIII, fig. 5, represents the occipital lobe in Cynocephalus porcarms. 

 The only noticeable changes are the increasing undulation of the inferior occipital 

 fissure and the appearance of two small sulculi, 1 and 2. 



It will be noticed that the secondary arch, O 1 ' and O 2 ', divides the lateral 

 surface of the occipital lobe, as I have defined it, into three distinct areas, and 

 these I shall call respectively the gyrus occipitalis primus (superior), the gyrus 

 occipitalis secundus (middle), and the gyrus occipitalis tertius (inferior), indicating 

 them as S. Oc, M. Oc and I. Oc. It will be found that these three convolutions, 

 separated by the two branches of the secondary occipital arch, can be traced 

 through all their developments up to the brain of Man, and I think I will be able 

 to show conclusively that if the occipital lobe be viewed in this way, most of the 

 difficulties met with in studying this region in human brains disappear. 



