Irving Hardesty 333 



The Appearance of the Medullary Sheath in the Spinal Cord 

 AND THE Occurrence of the Seal-ring Cells. 



Most of the observations upon the origin, development and structure 

 of the sheaths of the nerve fibers have been made upon the fibers of the 

 peripheral nerves. This is perhaps due to the greater ease with which 

 peripheral fibers ma}' be studied. Satisfactory fixation is easily obtained 

 with bits of peripheral Jier-ves, their fibers are more easily isolated, being 

 supported and separated by abundant connective tissue, and they have 

 thicker and stronger supporting sheaths than the fibers of the central 

 system. Indeed, the fibers of the central nervous system are described 

 as having no primitive sheaths, or sheaths of Schwann, at all. 



Notwithstanding various conflicting theories, the general concensus 

 with regard to the origin of the nerve axones and their medullary sheaths 

 may be summed up in the following: 



1. All axones arise as outgrowths or processes of " nerve cells." * 



2. Developing axones become invested by special cells which give rise 

 to the sheath of Schwann but probably have nothing to do with the forma- 

 tion of the myelin of the medullary sheaths. 



3. In development, the axone precedes, secondarily the sheath of 

 Schwann appears upon it, and lastly, or simultaneously with the begin- 

 ning of the sheath of Schwann, the myelin sheath begins to appear. 



4. Xo sheath of Schwann and therefore no sheath cells are described 

 for the nerve fibers of the central nervous system. In the peripheral 

 system the sheath cells and therefore the sheaths of Schwann are of 

 mesodermal origin. 



5. In the order in which they have been advanced, the theories of the 

 origin of the myelin sheath are: (a) that the myelin is formed through 

 the agency of the sheath cells by a process something like that by which 

 fat is formed by the fat cells (Ranvier) ; (b) that the m5'elin is formed 

 at the expense of the outermost portion of the nerve axone (Kolliker) ; 

 (c) that the myelin is of exogenous origin, formed in the blood and dis- 

 tributed from the blood-vessels to the axones (Wlassak) ; (d) that the 

 myelin arises as the result of influences exerted by the axone upon the 

 surrounding stroma (Bardeen). By "stroma" Bardeen refers to an 

 apparent fluid substance enclosed about the axone by the already formed 

 sheath of Schwann. The theory of Eanvier (supported by Vignal and 

 others) is invalidated by the statement that there are no sheaths of 

 Schwann in the central nervous system, wliile there are medullary 



' See Kolliker: Ueber die Entwickelung der Nervenfasern. Anat. Anz., 

 July, 1904, page 7. 



