ix.] THE NERVOUS SYSTEM. 383 



convolutions (commonly reckoned as the cerebellum) and ex- 

 tending in part into the cavity of the segment of the brain 

 in front of it. In many Teleostei (e.g. the Conger) the cor- 

 responding structure may consist partly of a fold extending 

 into the cavity of the more anteriorly situate cranial vesicle, 

 partly of a great smooth median prominence behind it. 



1 5. The CEREBELLUM of man, like his cerebrum, greatly ex- 

 ceeds the normal development of the part in Vertebrates 

 generally, thus differing from the intermediate brain seg- 

 ments which, as we have seen, are in him below the average 

 relative development of the same parts in Vertebrates. 



As we descend from man we find an increase of the 

 median portion of the cerebellum (the vermis) with a de- 

 crease of its lateral lobes, while the flocculus on each side 

 becomes prominent, and is (as we have seen) received in a 

 special fossa on the surface of the petrous bone. 



FIG. 337. DORSAL ASPECT OF BRAIN OF A PIGEON (Columba livia). 



2, one of the cerebral hemispheres ; 3, pineal gland ; 4, one of the optic lobes 

 (here lateral and depressed) ; 5, cerebellum. 



Complex folds, forming an arbor vita, are common to 

 man's class, to Birds, and to the Crocodiles, but are wanting 

 in lower forms. 



The cerebellum, even in the highest Ichthyopsida the 

 Frogs and Toads is reduced to little more than a transverse 

 band ; and it may be that it is hardly larger in Fishes, if, 

 as has been asserted, it forms but the lower part of that great 

 convex mass which has been usually described as the cere- 

 bellum, and which in the Elasmobranchs is so large and 

 convoluted, and is large but smooth in the Teleostei. If, 

 however, as is most probably the case, the convex mass is the 

 cerebellum, then this organ, after dwindling in size in Batra- 

 chians, once more regains its importance as we descend to 

 the lowest forms. 



Transverse fibres connecting the two lobes of the cerebel- 

 lum on the ventral side of the medulla are wanting in Birds, 



