30 ANATOMY OF THE CENTHAL NEBTOUS SYSTEM. 



in between the pyramidal cells from above and from below. These terminal 

 bushes (Endbaumchen) are well adapted to join together the elements of 

 the layer in which they terminate. 



The nerve-fibers of the brain and spinal cord are of very varying width 

 and in grown mammals probably all are provided with medullary sheaths. 



Every nerve-fiber loses its sheath of Schwann where it enters the central 

 system. Only a thin layer, present even in peripheral nerves and first seen 

 by Eanvier, covers the axis-cylinder within the brain and spinal cord. 



These are the elements from which the central nervous system is 

 constructed. 



In a general way, those parts which are composed principally of 

 medullated nerves appear white (white substance); those in which ganglion- 

 cells, axis-cylinders, and neuroglia predominate appear gray (gray sub- 

 stance). The gray substance is more vascular than the white. 



