296 MICROSCOPIC ANATOMY OF THE ORGANS. 



several cells of Purkinje. The smaller cells are multipolar, 

 and lie in the outer part of the molecular layer. 



Besides these nerve-cells which we have described, there are 

 in the cortex of the cerebellum nerve fibres which are for the 

 most part medulla ted and extend into the white substance. 



The white substance or medulla of the cerebellum consists 

 of medullated nerve fibres. The bundles are made up of axones 

 from the Purkinje cells, as well as fibres which arise from cells 

 in other parts of the nervous system, and enter the cerebellum 

 through one of the three peduncles. Certain special fibres have 

 been described by Ramon y Cajal as mossy fibres. These seem, 

 however, to represent only a stage in the life of other cells in 

 the granular layer. At their ends and where division takes 

 place there are mossy swellings. 



The ncuroglia is studied best in Golgi preparations. The 

 elements are quite similar to those of the cerebrum. The ar- 

 borescent cells are situated with the cell body between the gran- 

 ular and molecular layers. The cells with long rays are found 

 especially in the white matter (Fig. 221); while those with 

 short rays occur in the gray substance. 



By Weigert's method we learn that the glia fibres do not 

 form a dense network in the outer layers, as in the spinal cord 

 and cerebrum. On the contrary, we find in the molecular layer 

 radial fibres running from the surface into the deeper parts. 

 Only a few transverse fibres occur, which often in the region 

 of Purkinje's cells form a somewhat thick plexus. In the gran- 

 ular layers the glia fibres are very scarce, while in the med- 

 ullary substance they form a rich neuroglia network. 



E. Membranes Covering the Central Nervous System (Meninges). 



Enclosing the brain and spinal cord there are three mem- 

 branes : the outermost, known as the dura mater ; the middle, 

 the arachnoidea ; and the innermost, the pia mater. 



The dura mater spinalis is a membrane made up of con- 

 nective-tissue bundles containing fine elastic fibres. Both 

 surfaces are covered with ;i Inyrr of endothelial cells. 



The dura mater cerebrals is made up of two layers, an inner 



