MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 293 



some of the higher Simiadas the posterior branch of the fissura tri-radiatus becomes 

 separated, as we have seen, by the development of the small annectant or bridg- 

 ing convolution known as the gyrus dentatus. The arrangement of the fissures 

 remains, however, unchanged. 



In the Lemurs these fissures are present as well marked depressions, as may 

 be seen in Lemur nigrifrons, PI. XXXIV, fig. 17; in Indns, fig. 8, and Propit he- 

 ats Edwardsii, fig. 23. In a few forms, however, such as Avahis, fig. 20, the calca- 

 rine appears to be absent, PI. XXXVIII, fig. 8, shows them in Chrysothrix scutreus. 

 In Plate XXXVII, figs. 5, 7, 14, 17 and Plate XXXVIII, fig. 12, they may be 

 studied as they exist in the Simiadre as a class, whilst Plate XXXIX, fig. 3, shows 

 the relations as found in the chimpanzee as an example of the character of their 

 appearance in the Anthropomorpha, Plate XLI, figs. 2, 5, 6; XLII, 2, and XLIII, 3 

 and 4, show them as in Man. 



LOBES OF THE BRAIN. 



By means of these primary and fundamental fissures the brain may be di- 

 vided, as we have already seen, into three lobes : a posterior or occipital, and two 

 anterior, the occipito-frontal and occipito-temporal. It now becomes necessary to 

 seek for a more definite boundary separating the occipital from the other lobes. 

 We here approach, perhaps, the most confused subject in the whole range of 

 anatomy. The occipital lobe has been defined to be that part of the hemisphere 

 posterior to the parietal and temporal lobes, a rather indefinite division, 

 since the posterior boundaries of these lobes are not given hy the writers 

 who make this statement. It is relatively smaller in Man than in the 

 Simians and is also much better defined in them from the rest of the hemi- 

 sphere than in the human brain. We shall first consider its relations in the 

 monkeys, leaving the consideration of its proper boundaries in Man until we 

 come to treat of the convolutions found on its surface. The fissura occipitalis- 

 interna (parieto-occipital) , fig. 2, Pate XXXIV, 3, has been considered by most 

 writers as separating on the mesial surface the occipital from the parietal lobe, 

 but in Man this fissure ends at the lateral upper border of the hemisphere, being 

 surrounded by a small outwardly arched convolution, fig. 1, Plate XL, shown in 

 left hemisphere at 2. 



If we examine the brain of a monkey, such as Macacus, Cebtis, Cynocepha- 

 lus, etc., Plates XXXVII and XXXVIII, we will not find the fissura perpendic- 

 ularis ending in this way, but it becomes directly continuous with a long lateral 

 fissure, O 1 , the so-called external perpendicular fissure (fiss. perpendicularis ex- 

 terna). In the higher apes as a rule, however, such as the chimpanzee, fig. 13, 

 Plate XXXVIII, right hemisphere, and also in Ateles, fig. 15, PI. XXXVII, the 

 fissure will be found cut off in the same manner as in Man, by the convolu- 

 tion 2. Often this occurs in one hemisphere only, as is the case in the two speci- 

 mens referred to. The occipital lobe is, therefore, very well marked off in most 

 monkeys from the parietal by means of this fissure. The portion of the occipi- 



