372 



BIOLOGY. 



[BOOK vr. 



The posterior brain becomes the cerebellum. 



The surface of the cerebral hemispheres, at first completely 

 smooth in the embryon of the superior vertebrates and in the 

 inferior vertebrates (marsupials, edentals, &c.), goes into folds in 

 proportion to the evolutive or hierarchical progress. In most of 



Em Dry on of dog seen from the back, with 

 outline of the central nervous system, of 

 which the medullary sheath (6) forms a 

 furrow open above. Three primitive 

 vesicles (a) form the same number of 

 expansions ; the posterior part of the 

 marrow enlarges into the rhomboidal 

 sinus (of) ; c, lateral sheaths limiting 

 the outline of the body ; d, germinative 

 vestment external and msdian ; /, mu- 

 cous vestment. 



FIG. 63. 



Vertical sections through the brains 

 of some vertebrates : A, young 

 selacian (heptanchns) ; B, embryon 

 of adder ; C, embryon of she-goat ; 

 a, anterior brain ; 1), intermediary 

 brain ; c, median brain ; d, pos- 

 terior brain ; e, terminal brain ; s, 

 primitive cleft; h, hypophysis. 



the mammifers it is moulded into cords sinuous, hemicylin- 

 droidal, juxtaposed, called cerebral circumvolutions. 



The circumvolutions, gathered always into regular groups, are 

 usually so much the more flexuous, so much the more compli- 

 cated, the more the animal is intelligent. They are, for instance, 

 very richly developed in the elephant, the arthropoid apes, and. 



