278 ON THE RELATIONS OF MAN 



ment of the cerebral hemispheres and of the cerebellum, 

 but especially of the former, in respect to the other parts 

 of the brain. 



In the lower placental mammals, the cerebral hemi- 

 spheres leave the proper upper and posterior face of the 

 cerebellum completely visible, when the brain is viewed 

 from above, but, in the higher forms, the hinder part of 

 each hemisphere, separated only by the tentorium (p. 281) 

 from the anterior face of the cerebellum, inclines back- 

 wards and downwards, and grows out, as the so-called 

 " posterior lobe," so as at length to overlap and hide the 

 cerebellum. In all Mammals, each cerebral hemisphere 

 contains a cavity which is termed the ' ventricle,' and as 

 this ventricle is prolonged, on the one hand, forwards, 

 and on the other downwards, into the substance of the 

 hemisphere, it is said to have two horns or ' cornua,' an 

 ' anterior cornu,' and a ' descending cornu.' When the 

 posterior lobe is well developed, a third prolongation of 

 the ventricular cavity extends into it, and is called the 

 " posterior cornu." 



In the lower and smaller forms of placental Mammals 

 the surface of the cerebral hemispheres is either smooth 

 or evenly rounded, or exhibits a very few grooves, which 

 are technically termed ' sulci,' separating ridges or ' con- 

 volutions ' of the substance of the brain ; and the smaller 

 species of all orders tend to a similar smoothness of brain. 

 But, in the higher orders, and especially the larger members 

 of these orders, the grooves, or sulci, become extremely 

 numerous, and the intermediate convolutions proportion- 

 ately more complicated in their meanderings, until, in the 

 Elephant, the Porpoise, the higher Apes, and Man, the 

 cerebral surface appears a perfect labyrinth of tortuous 

 foldings. 



Where a posterior lobe exists and presents its customary 

 cavity the posterior cornu it commonly happens that a 

 particular sulcus appears upon the inner and under sur- 

 face of the lobe, parellel with and beneath the floor of the 

 cornu which is, as it were, arched over the roof of the 

 sulcus. It is as if the groove had been formed by indenting 

 the floor of the posterior horn from without with a blunt 

 instrument, so that the floor should rise as a convex emi- 

 nence. Now this eminence is what has been termed the 

 ' Hippocampus minor ' ; the ' Hippocampus major ' being 

 a larger eminence in the floor of the descending cornu. 



