U MAMMALIA: BRAINS 133 



sphere, it is said to have two horns or "cormia," 

 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 " convolutions " 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 proportionately 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 com- 

 monly happens that a particular sulcus appears 

 upon the inner and under surface of the lobe, 

 parallel 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 with- 

 out with a blunt instrument, so that the floor 

 should rise as a convex eminence. Now thia 



