MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 285 



which hare been considered as fundamental and establishing the morphological 

 type, such as the fissura centralis (fissure of Rolando), the ramus anterior ascen- 

 dens of the fissura SyMi. the fissura perpendicularis internus (parieto-occipital), 

 etc., have not the significance that has been attached to them, but belong to the 

 class of secondary fissures which Ave have not as yet discussed. It is true that in 

 Man and most of the Primates these fissures attain great importance as respects 

 depth, form and constancy of development. This, however, is due to special condi- 

 tions of groAvth and not to any increased morphological significance. I regard, for 

 instance, the fissura centralis in the same light and as of the same importance mor- 

 phologically as the fissura frontalis, superior et inferior and the fissura temporalis 

 superior et inferior, etc. To the fissura parieto-occipitalis may be ascribed a some- 

 what deeper significance. The reasons which haA r eled me to these conclusions will 

 be giA r en in another place. 



FISSURA SYLVII. 



The fissure or fossa of Sylvius is the most constant and important of all of the 

 fissures of the hemisphere, and may be recognized in the brains of all the Primates 

 as a typical, well marked fissure. With regard to the fundamental and primary 

 nature of this fissure there can be no doubt, and on this point all AAriters are agreed. 

 According to Pansch. it is, hoAA-eA-er. absent in Chiromys, at least in the sense in 

 which it exists in the other Primates ; that is, there is no Island of Reil coA'ered by 

 an OA r ergroAA r ing mantel or operculum. In Plate XXXIV, fig. 24, may be seen the 

 brain of the Aye-Aye, Chiromys, and a fissure. S., Avhich at least represents in appear- 

 ance and position the fissure of Sylvius. Pansch, from this and other facts, does 

 not belieA*e that the Aye-Aye should be classed AA r ith the Primates, but that it be- 

 longs to another group principally constituted by the Carnivora. With this excep- 

 tion it is uniformly present, together with an accompanying operculum which co\ r ers 

 the Island of Reil. 



It is found distinct and aacII marked in the brain of Man at the end of the 

 third month. Plate XXXV, figs. 1 and 2. The Sylvian is here seen as a broad de- 

 pression or fossa. S.S.. extending backAA r ard from the place of entrance of the cerebral 

 peduncle, C.P.. to a position midway betAA r een this point and the posterior part of 

 the hemisphere. It serves, even at this earl}- date, to diAnde the hemisphere into 

 two parts, an occipitofrontal. O.F., and occipitotemporal, O.T. The floor of this 

 fossa at its anterior part becomes the future Island of Reil or lobulus centralis, 

 Avhich corresponds internally with the position of the corpus striatum. The two 

 portions, 0. F. and 0. T., groAA- fonvard as deA r elopment adA^ances, in such a manner 

 that the AA r hole hemisphere appears bent into an arch around the point of entrance 

 of the cerebral peduncle. 



As deA r elopment advances, this fossa gradually deepens and also AA'iclens at its 

 anterior portion. From the material at my command it seems to attain its maxi- 

 mum breadth at about the middle or toward the end of the fifth month. In the 

 brain of a foetus at the middle of the fifth month, hardened in chloride of zinc, I 



