CHAP. XVI.] SOME OTHER MAMMALS. 275 



External Topography of the Brain in Quadrupeds, 

 and some other Mammals. 



The thickness of the layer of ganglionic Grey Matter 

 on the surface of the brain undergoes a gradual increase 

 among the Yertebrata. The layer is so thin in Fishes 

 that the surface of the Cerebral Lobes appears almost 

 white to the naked eye. In Mammals, however, we have, 

 even in the lowest of them (and its thickness increases in 

 higher forms) a continuous stratum of such matter cover- 

 ing the whole of the Cerebral Hemispheres. Of course 

 the more the surface of the hemisphere is folded and con- 

 voluted, the greater is its proportional amount, since this 

 Grey Matter covers all parts of the surface, whether it be 

 folded inwards or outwards (fig. 85, c, c). 



In Fishes, Amphibia, Eeptiles, and Birds, there are no 

 regular 'fissures,' and, consequently, no division of the Cere- 

 brum into ' lobes. ' Each Cerebral Hemisphere has, indeed, 

 in these lower forms been supposed by some, though on 

 insufficient grounds, to correspond with the ' anterior lobe ' 

 of the brain of the Ape and Man. The ' middle lobes ' 

 are believed to make their appearance subsequently, as 

 added parts, in the lower Quadrupeds ; while the ' pos- 

 terior lobes ' are, similarly, deemed to make their first 

 appearance among the lower Quadrumana. But, as Prof. 

 Marshall very properly observes, "the lobes may not be 

 distinguishable, and yet homologous parts of the cerebral 

 hemispheres may be present, however slightly developed, 

 throughout all the Vertebrates." The appearance, indeed, 

 of the brain of some of the Cetacea, such as the Porpoise 

 and the Dolphin, makes it rather more probable that it is the 

 middle regions of the brain which are specially developed 

 in them, while both anterior and posterior lobes (and 



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