128 ANATOMY OF VERTEBRATES. 



' We are guided to the homologous parts of the cerebral hemi- 

 spheres throughout their range of developement in the Mammalian 

 class, in a great measure, by their relations to other parts of the 

 brain. The portions more immediately surrounding the cerebral 

 crura, 1 those which overarch the corpora striata and thalami and 

 overlie the olfactory crura, or at least their beginnings, can 

 hardly be doubted to be corresponding parts in all Mammals. The 

 inferior prominences behind the " crura rhinencephali," forming 

 the " protuberantiae natiformes" of some anthropotomists {b f basi- 

 rhinal fold), the inverted hippocampal fold, its labia and fissure, 

 are plainly determinable throughout the class, as is also the 

 sylvian fissure, 5, somewhat less constant, dividing the part of 

 the hemisphere terminated by/', figs. 113 and 115, and sometimes 

 called " inferior lobe," from the part which is in front of it : the 

 superadded cerebral substance to the above more constant parts 

 of the hemispheres is that which, in Man, advances, overlaps, 

 and extends beyond the olfactory lobe, and that which extends 

 backward in like relation to the cerebellum. 



6 If one can predicate homology of any folds or fissures of the 

 cerebral superficies, throughout the Mammalian class, it must 

 be at the above-defined middle part of the more developed 

 hemispheres, and especially at those fissures, viz. 2, ectorhinal, 4, 

 hippocampal, 5, sylvian, 7, callosal, 7', supercallosal, that are the 

 most constant throughout the series. The upper surface of the 

 hemispheres, as we have seen, is subject to different ways of 

 folding : in Echidna the plaits go across, in Felts along it, while 

 in Bos and Simia they run askew, yet contrariwise ; in one 

 from behind forward and inward, in the other forward and out- 

 ward. It may seem, to some, that each leading division of 

 Gyrencephala should have its own system of nomenclature and 

 symbolism of brain-folds — that homologous convolutions can only 

 be satisfactorily determined within the limits of such groups as 

 Ungulata, Unguiculata, Quadrumana. In a degree this is true ; 

 the grounds of homology are such in regard to some folds (6 and 

 7') as to leave room for difference of choice ; but there are others 

 that have a surer basis for homologising. Take, for example, the 

 " sylvian fissure," 5 : the fold e, that immediately overarches and 

 forms it, is determinable : one part of the fold forms the anterior, 

 the other the posterior, lip of the fissure : they are united or 

 continuous by the overarching part in most Unguiculates and 

 Ungulates. The homology of the sylvian fissure and fold is not 



1 Subsequently defined as ' prosencei>hal:c.' 



