Irving Hardesty 353 



Summary. 



1. There are present upon the medullated fibers of the spinal cord cells 

 similar to the nerve corpuscles or sheath cells of the peripheral nerve 

 fibers. 



2. These cells are more abundant and possess more protoplasm during 

 the period of the most active formation of the myelin sheath than during 

 the later stages. 



3. They do not appear upon the fibers till after the fibers have begun 

 to acquire myelin. 



4. They are apparently derived from the nuclei and protoplasm of the 

 syncytium of the developing spinal cord and are perhaps difi:erentiated 

 through some influence exerted upon the syncytium by the developing 

 myelin upon the axones. 



5. These cells occur much more rarely upon the adult fibers and when 

 found possess little or no protoplasm. 



6. The framework of the medullary sheaths of the spinal cord occurs 

 in the form of a lamellated reticulum in the meshes of which the myelin 

 is supported. 



7. This framework difl'ers from that of the medullary sheaths of the 

 peripheral nerve fibers in that it is not quite so heavy and always shows 

 an arrangement parallel with the axone of the fiber. 



8. The more or less parallel arrangement of the reticulum Is probably 

 the normal condition of the framework in the peripheral nerves also, the 

 post-mortem appearance of the usually described coarse " neurokeratin 

 network " being but a distortion of the normal arrangement produced by 

 a continued coalescence of the much finer globules of the original myelin 

 emulsion, the occasional conical arrangement of the framework repre- 

 senting the final result of the further coalescence of the globules. 



9. The framework of the medullary sheaths of the spinal cord resists 

 digestion as does that of the medullary sheaths of the peripheral nerves. 



10. While there is a supporting contingent of white fibrous tissue 

 among the nerve fibers of the spinal cord, the statement is confirmed that 

 there is no distinct, separate membrane investing the fibers of the central 

 system corresponding to the sheath of Schwann investing the medullated 

 fibers of the peripheral nerves. 



11. The sheath cells of the spinal cord are probably concerned in the 

 development of the framework of the medullary sheath and probably in 

 a manner similar to that in which the other fibrous supporting tissues of 

 the body are developed. 



