MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 301 



in degree of development to the Old World macaeques, Plate XXXVII, figs. 1, 

 5, 6, 7, 8, 9 and 14, and also similar to the human foetal brain at the end of the 

 eighth month. In Ateles, Plate XXXVII, figs. 17 and 18, we have a still nearer 

 approach to the human type, whilst in the Anthropomorpha, including the gib- 

 bons, orang, chimpanzee and gorilla, the brain becomes almost identical with the 

 brain of the human foetus of a little past the middle of the ninth month. 



OCCIPITOFRONTAL LOBE. 



The boundaries and connections of this lobe have already been given. It 

 occupies that part of the cranial cavity formed by the frontal and parietal bones, 

 the lower part of its anterior extremity resting upon the orbital surface of the 

 frontal bone, known by the name of the orbital surface. It presents besides this, 

 two other surfaces : a mesial, corresponding to the mesial of the occipitotem- 

 poral and a supero-lateral curvilinear surface similar to the infero-lateral of the 

 same lobe. We will first call attention to the structure and relations of the mesial 

 surface. 



Occipitofrontal Lobe (Mesial Surface). 



On the mesial occipito-frontal surface two convolutions separated by a fis- 

 sure m. o. f. are found. This has been termed by Huxley, Bischoff, Turner, 

 Marshall, Ecker, etc., the fissura calloso-marginalis, and by Pansch the sulcus 

 medialis-fronto-parietalis, the grand sillon du lobe fronto-parietal of Gratiolet. 

 The posterior termination of this fissure is, according to all previous authorities, 

 at the point marked 16, PL XXXIV, fig. 2, and c. m. in various figures of plates 

 showing the mesial surface. 



This point of termination I cannot consider as the proper morphological 

 extremity of this fissure, but believe that its real termination is farther back, 

 consisting of what is usually considered as a sulcus constantly found on 

 the so-called lobulus praecuneus. This is an elongated H-shaped fissure 

 marked m. o. f. 1 on the plates. It is true that in the higher Primates 

 this fissure is usually separated from the anterior portion, the calloso-marginal, 

 but this fissure itself is very seldom a continuous one in these animals, 

 being generally divided into a number of short branches as may be seen in 

 PL XXXIX, fig. 3, PL XLI, fig. 2, and PL XLII, fig. 2. I think it will be 

 perceived at once that this sulcus of the prsecuneus, m. o. f. 1 is similar in all 

 respects to the other H-like separations of the calloso-marginal, and ought, there- 

 fore, to be considered as a part of it. This fissure as a whole is one of the most 

 distinct fissures of the hemisphere and was even recognized as constant by Vicq 

 d'Azyr. I propose for it, therefore, a name in consonance with the corresponding 

 fissure of the occipito-temporal mesial surface, and will call it the mesial occipito- 

 fontal fissure, (fissura occipito-frontalis medialis). 



It appears in the human foetus a short time previous to the appearance of 

 the mesial occipito-temporal fissure and may be distinctly recognized at the middle 



