THE BRAIN 1 53 



tinuity being evident in the lower vertebrates, but somewhat ob- 

 scured in the mammals by the development of the pons, soon to be 

 mentioned. 



Cerebelliun. — The cerebellum or metencephalon is developed 

 from the anterior part of the dorsal zone and roof plate of the hind- 

 brain, the roof plate being invaded by nervous matter from either 

 side. In front it is bounded by the isthmus, which, together with 

 the anterior wall of the cerebellum, forms a transverse fold, the 

 anterior medullary velum (valve of Vieussens), which dips into the 

 fourth ventricle. In the lower vertebrates there is but a single part 

 to the cerebellum, which corresponds to the vermis of the mammals. 

 In the higher reptiles and birds a small lateral outgrowth on either 

 side, the flocculus (fig. 179) appears near the ventral portion. In 

 the mammals a large cerebellar hemisphere is developed on either 

 side, between vermis and flocculus, the latter being thus forced to the 

 lower surface. 



In the lower vertebrates a few fibres from either side of the cere- 

 bellum pass across the ventral surface of the pyramids to reach the 

 other side. In the mammals the number of these is greatly increased, 

 so that a large transverse band, the pons (Varolii) is developed on the 

 lower surface of this part of the brain stem. These fibres, as they 

 enter or emerge from the cerebellum, form the middle pedimcle or 

 brachiimi pontis. In front, the cerebellum is connected with the 

 mid-brain by a longitudinal tract, the anterior peduncle or brachitmi 

 conjimctivum. This starts, in the mammals, from a large nerve 

 centre, the nucleus dentatus, in either cerebellar hemisphere. 



Mesencephalon. — The mesencephalon is practically the mid- 

 brain of the early embryo. It undergoes little modification except 

 that, on the dorsal surface, two lateral swellings, the optic lobes 

 are developed, these being transversely divided in the mammals into 

 four, the corpora quadrigemina (fig. 167, ol). On either side the 

 optic lobe is connected with fibres coming from the eye of the opposite 

 side. The floor is greatly thickened by the fibres of the pyramidal 

 and cerebral peduncular tracts, and the floor plate disappears. In 

 the higher groups the primitive ventricle of the mid-brain has been 

 reduced to the aqueduct, but in the fishes it remains large and ex- 

 tends, as the epiccele, into the optic lobes (fig. 174). At the anterior 

 end of the dorsal part of the mid-brain, there is a band of nerve fibres 

 which crosses from one side to the other (fig. 160, pc). Such cross 

 connexions are called commisstires and they serve to connect the two 



