114 THE RELATIONS OF MAN 



spheres and of tlie cerebellum, but especially of tbe for- 

 mer, 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. 117) 

 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 hemi- 

 sphere, it is said to have two horns or ' cornua,' an ' ante- 

 rior cornu,' and a ' descending cornu.' When the poste- 

 rior lobe is well developed, a third prolongation of the 

 ventricular cavity extends into it, and is called the " pos- 

 terior 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 mem- 

 bers of these orders, the grooves, or sulci, become ex- 

 tremely numerous, and the intermediate convolutions pro- 

 portionately 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 custom- 

 ary cavity — the posterior cornu — it commonly happens 



