THE BRAIN 229 



optic lobes subdivides them into an anterior larger and a posterior 

 smaller pair of lobes (cf. p. 221). 



The division of the large cerebellum into a median and two 

 lateral portions, already indicated in Reptiles, is carried to a still 

 further extent in Mammals. The median portion gives rise to the 

 so-called superior vermis, while the lateral parts form the lateral 

 lobes and flocculi (Figs. 170, 171). In Carnivores, certain 

 Edentates, Pigs, and Lemurs, the vermis is relatively large as 

 compared with the lateral portions ; while in Cetaceans, Elephants, 

 Apes, and Man the latter are more highly developed and the 

 median lobe reduced. The two lateral lobes of the cerebellum are 

 connected by a large commissure, the pons Varolii : this extends 

 round the medulla oblongata ventrally, and is more largely 

 developed the higher we pass in the mammalian series. Other 

 bands of nerve-fibres connecting the cerebellum with various parts 

 of the brain are spoken of as the anterior, middle, and posterior 



EM 



LCa* 



JfH 



Caeb 



Fio. 174. DIAGRAM OF THE CHIEF SYSTEMS OF FIBRES OF THE MAMMALIAN 

 BRAIN. (From a drawing by A. Ecker.) 



Cnc, crura cerebelli ad corpora bigemina ; Cacb, crura medulla ad cerebellum ; 

 Cap, crura cerebelli ad pontem ; (7(7, crura (pedunculi) cerebri ; C8, corpus 

 striatum ; HH, cerebellum ; HM, hemisphere ; Z/, lemniscus : P, pons ; 

 Th, optic thalamus. 



peduncles of the cerebellum, the relations of which, and of the 

 crura cerebri, are indicated in Fig. 174. 



A study of the brain-casts in certain North American Eocene 

 forms is very instructive from an evolutional point of view, and 

 shows that the brain, and more especially the fore-brain, in these 

 animals was of extremely small size relatively (Fig. 175). The 

 brain of Dinoceras mirabile might easily be mistaken for that of 

 a Lizard, and was so small that it could easily be drawn through 

 the greater part of the neural canal : in the Cretaceous Dinosaurian 

 Triceratops, the brain was apparently still smaller relatively. The 

 olfactory nerves were extremely well developed in these forms. 



In connection with the importance of the brain in modelling 

 the form of the skull, it may be mentioned that in many 

 Mammals, including Man, the outer surface of the skull in various 



