THE NEUROGLIA. 435 



wander from their position near the neural canal toward the per- 

 iphery of the medullary tube, where they develop into neuroglia 

 cells. 



Owing to the fact that of the several methods now at hand for 

 studying neuroglia tissue no two give identical results, the views 

 concerning this tissue are still at variance. The Golgi or chrome- 

 silver method was for many years the only method by means of 

 which the elements of neuroglia tissue were brought to light with 

 any degree of clearness. In preparations of the central nervous 

 system treated with this method all the neuroglia elements appear 

 as cells with processes. The cell bodies of these cells as also the 

 processes being stained black or nearly black (as seen with trans- 

 mitted light) so that the relations of the processes to the cellular 

 constituents can not be ascertained, investigators who have made use 

 of this method in their study of neuroglia distinguish two essentially 

 different cellular elements of the neuroglia: ependymal cells, pre- 

 viously mentioned, and neuroglia cells, so-called spider cells or 

 astrocytes. The astrocytes are grouped under two main heads : 

 short-rayed astrocytes, possessing a few short processes, found in the 

 gray matter, and long-rayed astrocytes with many fine and long pro- 

 cesses, which do not appear to branch, found both in the gray and 

 white matter. The two types of astrocytes are not clearly defined, 

 as intermediate types are also found. In figure 349 are shown two 

 astrocytes (long-rayed) as seen in chrome-silver preparations. 



A number of investigators have in recent years perfected methods 

 by means of which neuroglia tissue could be stained differentially 

 Weigert, Mallory, Benda. In tissues treated after any one of these 

 rather complicated differential staining methods the processes of the 

 neuroglia cells as seen in chrome-silver preparations appear in the 

 form of well-contoured fibrils, which are not interrupted by the cell- 

 bodies of the neuroglia cells, from which they are either entirely 

 separated or are seen to pass through the protoplasm of the cells 

 without losing their identity. In preparations of the central nervous 

 system stained after Benda' s differential neuroglia tissue staining 

 method, numerous neuroglia cells may be observed both in the gray 

 and white matter. Certain of these cells possess very little proto- 

 plasm, others and these are in the majority present it to an ap- 

 preciable extent. The shape of such cells varies. When situated 

 in the main mass of the white matter of the spinal cord, and seen in 

 cross-sections of the cord, they present an irregular triangular and 

 quadrangular form, with protoplasmic branches which arise from the 

 angles and which extend for a variable distance between the nerve- 

 fibers. In such preparations it may be seen that the neuroglia 

 fibers pass in close proximity to the neuroglia cells, apparently em- 

 bedded in the outermost part of their protoplasm, and often follow- 

 ing the protoplasmic processes. This view of the structure of neu- 

 rogliar tissue is more in accord with recent investigations on this 



