Irving Hardest}' 341 



!>pinal cord of the adult hog. They are all selected from teased prepa- 

 rations of the same specimen. Three of these pieces of fiber (a, h and c) 

 possess what may be reasonably considered sheath cells. They were 

 found after considerable search. Sometimes cells could be found adher- 

 ing to the fibers in the manner similar to that shown upon the fiber d, 

 but these were not considered as examples of the type sought. Cells 

 situated in indentations of the sheath and with a more or less even outer 

 contour were sought as more probable examples of the cell in mind. 



There are many fibers in the adult cord relatively larger than the 

 type d, but so far I have found no cells upon their sheaths, which latter 

 are always deeply blackened by the osmic acid. Of the smallest medul- 

 lated fibers in the adult cord, the types indicated by e and g are inter- 

 esting. In the peculiar bulbous enlargements of their sheaths and in 

 their relative size, they are identical with certain of the fibers described 

 by Ranvier in the spinal cord of the dog. Ranvier pictured a nucleus 

 with protoplasm about it adhering to what appears to be one of the larger 

 of this type of fibers. So lar I have not found examples of sheath cells 

 upon any of them in the adult hog, but this may be due to the fact that 

 such fibers are much less numerous in the cord than fibers of the larger 

 type and that therefore a much less number of them was examined. 

 Their peculiar appearance can hardly be considered wholly artifact, for 

 the type is quite constant and often one of them may be followed un- 

 broken for several millimeters and throughout shows the same form. It 

 would be difficult to determine whether or not they are younger fibers in 

 the process of mecluUation. They resemble certain of the Remak fibers. 

 The few undoubted collateral branches I have seen in the preparations 

 were of this general type of fiber. 



The relation the sheath cells bear to the medullary sheaths of the 

 central nervous system is as much of a question as it is in the peripheral 

 nerves. When, in the study of this question, one examines into the 

 nature of the medullary sheath in the spinal cord, it is immediately evi- 

 dent that it differs from that of the peripheral nerve fibers in several par- 

 ticulars. 



While the fiber of the spinal cord is, of course, devoid of any structure 

 similar to the capsule or sheath of Henle possessed by certain peripheral 

 medullated fibers, it also lacks a distinct sheath of Schwann. Many deny 

 that the nerve fiber of the central system possesses anything similar to the 

 sheath of Schwann found on the peripheral filier. Under certain con- 

 ditions when the myelin is crushed or shrunken away, there may be seen 

 occasionally evidences of a very delicate sheath al^out the periphery of 

 the medullary sheath (/). Fig. 6). jNEore usually, however, such must 



