240 NERVOUS SYStEM OF VERTEBRATES. 



cells, the small spines of which apparently serve for connections 

 with the fine fibers. The cells of the second type are much larger 

 than the granule cells, their dendrites spread in both granular 

 and molecular layers, and their neurites branch immediately 

 and profusely in the granular layer. 



The molecular layer contains two chief types of cells. In the 

 deeper part of the layer are numerous cells the behavior of whose 

 neurites has given them the name of basket cells. The neurites 

 begin as slender fibers which grow thicker as they run parallel 

 with the Purkinje cell layer. At intervals they give off collateral 

 branches which run down and branch to form basket-hke net- 

 works around the bodies of the Purkinje cells. Although such 

 cells have been seen in rare cases in the cerebellum of fishes, they 

 are not highly developed or numerous below the mammals. In 

 the outer part of the layer are numerous small cortical cells 

 whose neurites are short and either branch repeatedly close to the 

 cell or have a longer or shorter horizontal course before ending. 



Fiber tracts of the cerebellum. — ^The fibers connecting 

 the cerebellar cortex with other parts of the brain in part arise in 

 the cerebellum, in part end in it. Since the structure of the 

 cortex and the arrangement of the fiber endings are everywhere 

 the same, it follows that all three peduncles of the cerebellum 

 carry both in-coming and out-going fibers. The in-coming fibers 

 are of two forms: moss fibers, which bear peculiar bundles of 

 short branches in their course and at their ends, and fibers which 

 end by complex net-like end-branches. The moss fibers occasion- 

 ally bifurcate and give off frequent collaterals, so that the terminal 

 branches are widely distributed. The pecuhar small tufts of 

 end-branches stand in relation with the similar branches of the 

 dendrites of the granule cells. The second kind of fibers rise 

 through the granular layer, apply themselves to the surface of the 

 Purkinje cell dendrites and ramify upon the branches of these 

 dendrites (Fig. 122). There is a remarkable uniformity in the 

 character and arrangement of the nerve cells and in- coming fibers 

 in all parts of the cerebellar cortex in mammals. The incoming 

 fibers in mammals are very different from those in lower verte- 

 brates. In fishes the great majority of in-coming fibers are somatic 



