THE CENTRAL NERVOUS SYSTEM. 301 



appear in transverse section, which, singly or in bundles, pur- 

 sue a longitudinal direction. 



Amyelinic fibres are found everywhere in the gray matter 

 pursuing various courses. The gray commissure is composed 

 almost completely of amyelinic fibres connecting the lateral 

 gray masses. 



Besides the above-described elements the gray substance 

 seems to have a structureless or slightly granular matrix, in 

 which the other elements are embedded. In the study of the 

 spinal gray matter osmic acid and carmine preparations are by 

 far the most useful, although much is to be learned by the ex- 

 amination of fresh specimens teased in serum. To study the 

 cells, cut out a piece from the anterior horn of a fresh cord 

 and tease in serum. A preliminary treatment with osmic acid 

 or ammoniacal carmine is sometimes advantageous. 



The epithelium of tlie spinal cord lines the central canal. 

 The most internal layer is of the cylindrical, ciliated variety. 

 On account of the difficulty in obtaining fresh specimens the 

 cilia are never seen in the human cord, though they undoubt- 

 edly exist. The epithelial cells, as they do exist in the human 

 cord, have a square base, taper to a slender thread toward 

 the apex, which penetrates the layer of young, round epithe- 

 lial cells, and is lost in the granular central gray matter. The 

 cells of the second layer are round, granular, and thickly 

 crowded. 



The subepil7ielial tissue, for some distance around the cen- 

 tral canal, consists of embryonal cells in a granular matrix. 



The central canal 1 has no constant shape, varies greatly in 

 size, and is often choked with desquamated epithelium. Its 

 position, as its name indicates, is in the middle of the gray 

 commissure, on a line passing between the anterior fissure and 

 posterior septum. The general features of the spinal cord hav- 

 ing been pointed out, the peculiarities of different regions will 

 now be shown. 



Special study of the different portions of tlie cord. The 

 cord will be studied from below upward. The mode of study 

 will mainly be by sections made after Clarke's method. The 

 cord, after slitting the dura mater with the scissors up the 

 front and back, is cut in segments 3 ctm. long, which adhere 



1 Sometimes double. See Seguin: Am. Jour. Med. Sci., p. 427. 1872. 



