353 Sheath Colls and Axoue Sheaths in the Central Xervous System 



Basement membranes are of connective tissue origin and are not cellular. 

 The sheath nuclei may represent certain of the cells having to do with 

 the development of the sheath of Schwann which were enclosed within 

 the sheath and therefore separated from the similar nuclei distributed in 

 the endoneurium outside the sheath. It is well known that, while an 

 internode of the medullated peripheral fiber usually has a sheath nucleus, 

 one is not present in every case, and further, a single internode may 

 sometimes show two or more sheath nuclei. 



On the other hand the sheath nuclei may have to do with the develop- 

 ment of the framework of the myelin sheath as well as with the sheath 

 of Schwann. The framework, both of the central and peripheral fibers, 

 stains like the sheath of Schwann. It is suggested that the seal-ring 

 cells of the foetus and the sheath nuclei of the adult spinal cord repre- 

 sent " cells " derived from the syncytium and whose activities result in 

 the reticulated framework of the medullary sheath ; that, wherever found, 

 the protoplasm about the nucleus represents only the endoplasm which is 

 being transformed into exoplasm, which in its turn is transformed into 

 the lamellated reticulum by a process similar to that described by Mall 

 in the development of the fibrous connective tissues ; and finally, that the 

 origin of the framework and the origin of the sheath of Schwann of the 

 peripheral fibers may be similar. 



The framework of the medullary sheath in the spinal cord resists the 

 digestive action of pancreatin as first noted by Ewald and Kiihne for 

 that of the peripheral fiber. However, in the digested sections it does 

 not appear so abundant as after the Benda neuroglia stain. After fixa- 

 tion in other fluids it does not appear as abundant as it does after fixa- 

 tion with formalin and also it stains very lightly or not at all by the 

 ordinary staining methods. IMallory's method for white fibrous con- 

 nective tissue stains it but lightly. The digested sections of the spinai 

 cord stained by this method or with strong fuchsin solutions show more 

 or less collapsed circles representing the transversely cut nerve fibers. 

 These circles contain remnants of the framework usually so collapsed and 

 washed together that little semblance of the original arrangement can be 

 made out. Occasionally there is a small inner ring showing the opening 

 from which the axone has been digested and representing the inner layer 

 of the framework or axolemma with certain other portions of the frame- 

 work collapsed upon it. The outer ring, representing the periphery of 

 the fiber, is better maintained, due probably to the presence of the inter* 

 stitial framework of the white fibrous connective tissue of the spinal cord. 



