86 THE NERVOUS SYSTEM 



The neuroglia cells are differentiated from the spongioblasts. These, when 

 stained by the Golgi method, appear as small cells with many processes. Some 

 have long slender processes, the spider cells or long rayed astrocytes; others have 

 short thick varicose processes, the mossy cells or short rayed astrocytes (Fig. 

 61). Special neuroglia stains, like that of Weigert, show that an astrocyte is 

 composed of a glia cell associated with many glia fibers. Some authors main- 

 tain that the fibers run through the cytoplasm, while others assert that they 

 merely pass along the surface of the cell. In any case the fibers are to be re- 

 garded as products of these cells. Neuroglia cells and fibers are found every- 

 where throughout the gray and white matter of the spinal cord, forming a sup- 

 porting framework for the nervous elements. A special condensation of neu- 



Unmyelinated fibers 



inated fibers 



Fig. 62. From a cross-section through the spinal cord of a rabbit showing the structure of the white 

 matter as revealed by the Cajal method. (Cajal.) 



roglia surrounds the central canal and is known as the substantia gelatinosa 

 centralis. In addition to the neuroglia this contains some nerve-fibers and 

 cells. Beneath the pia mater and closely investing the spinal cord externally 

 is a thin stratum of neuroglia, the glial sheath, which dips into the cord along 

 with the pial septa. The posterior median septum is composed of neuroglia 

 and greatly elongated ependymal elements, and is in no part formed by the 

 pia mater. 



White Substance. The white matter of the spinal cord consists of longi- 

 tudinally coursing bundles of nerve-fibers, bound together by a feltwork of 

 neuroglia fibers in which are scattered neuroglia cells. A majority of the neu- 

 roglia fibers run in a direction transverse to the long axis of the nerve-fibers. 

 Blood-vessels enter the cord from the pia mater and are accompanied by con- 



