XERVOUS SYSTEM 859 



RHOMBENCEPHALON 

 THE CEREBELLUM 



The cerebellum or metencephalon (Figs. 472, 481) is the coordi- 

 nating organ growing behind the two cerebral hemispheres. The isthmus 

 which connects the mid-brain and hind-brain lies directly in front of the 

 cerebellum. The cephalic anterior wall of the cerebellum meets with the 

 isthmus to form a transverse fold known as the anterior medullary velum 

 (valve of Vieussens) which dips into the fourth ventricle. The median 

 ridge of the cerebellum is known as the vermis. This is the only part of 

 the cerebellum wnich the lower vertebrates possess. In some of the 

 higher reptiles and birds, however, a small outgrowth occurs on each 

 side called the flocculus and it is between the flocculus and the vermis 

 that the cerebellar hemispheres develop in the mammals. This pushes 

 the flocculus ventrad. 



Quite a number of fibers grow from one side of the cerebellum to the 

 other on the ventral side of the brain stem. This forms a large transverse 

 band called the pons or bridge. The lower vertebrates have only a few 

 fibers of this kind so that the pons is very narrow in them. There is a 

 groove or tract running longitudinally from the cerebellum to the mid- 

 brain along the side of this pons and these lateral tracts are called an- 

 terior peduncles, while the central or median tract is called the middle 

 peduncle or brachium pontis. The origin in the cerebellum of the an- 

 terior peduncle is called the nucleus dentatus. 



MEDULLA OBLONGATA 



This is really a large swelling between the cephalic end of the spinal 

 cord and the brain proper. Various marrow-centers appear in the ven- 

 tral side of the floor serving as centers by which and through which 

 efferent or outgoing fibers are redistributed to other nerve cells. The 

 head end of the medulla, by being expanded, forces the various fiber 

 tracts of the dorsal funiculi, as well as of the dorsal part of the lateral 

 funiculi, over to the side of the cerebellum where they enter, bending ab- 

 ruptly inward and forming a cord called the corpus restiforme, also 



Vtlum mednllare 

 tost. 



Ncdulns 



Leitulus gracilit 

 Lebvlvs semilvnaris 

 inftrior 



=^-^ 



Fig. 481. 

 Human cerebellum viewed from below and in front. (After Villiger.) 



