148 THE NERVOUS SYSTEM. 



Near the pars commusuralis is a transverse arched band of fibres, 

 passing from the under surface of one half of the cord over the 

 ventral longitudinal fissure through the septum medium to the under 

 surface of the opposite half ; part of the fibres are continued up- 

 wards along the periphery to the cerebellum, part to the nvcfc'iix 

 magnns. Vertical, straight, or slightly arched fibres are found in 

 the walls of the fourth ventricle. 



A section from the medulla oblongata has a larger amount of 

 pigment than a section from the spinal cord, and the anterior portion 

 of the medulla oblongata contains more than the posterior portion. 

 The pigment is chiefly found in a curved line, placed in the lower 

 and outer parts of the grey matter ; the amount present varies in 

 different specimens. 



b. The Cerebellum and Valvula cerebelli. 



a. External form. The cerebellum is a thin, semilunar plate, 

 which projects between the optic lobes and the fourth ventricle, 

 its base covering the most anterior part of the ventricle; the 

 posterior surface possesses a very faint median fissure (Reissner). 

 The valvula cerebelli (Velum medullare anteriws, Reissner) is the thin 

 lamella which connects the anterior surface of the cerebellum with 

 the optic lobes. 



/8. Minute structure. By means of longitudinal, vertical sec- 

 tions, the posterior surface of the cerebellum is seen to be covered 

 with epithelium ; in the lower part of the surface this is columnar 

 or conical, above it is flattened : immediately beneath, that is in 

 front of this is a finely granular layer, with very closely packed and 

 granular nuclei. In front of these is a stratum of nerve-fibres 

 forming the second layer of the cerebellum. 



Still more anteriorly is the third layer of the cerebellum, an 

 irregular double layer of large cells (PurkinjVs cells, Denissenko) ; 

 the cells have an average length and breadth of 0*040 mm. and 

 0*015 mm. respectively; they are pear-shaped or of spindle-form, 

 and possess usually two well-marked processes, one passing into 

 the layer behind, the other forwards into the anterior layer to be 

 immediately described, while other less distinct processes radiate 

 irregularly in all directions. The fourth and most anterior layer 

 of the cerebellum is a thick stratum of nerve fibres with numerous 

 nuclei (0*006 to o - oo8 mm. diameter). The fibres are for the 

 most part arranged transversely, but some course in various direc- 

 tions. These fibres underlie the flattened epithelium which covers 

 the anterior surface of the cerebellum. 



