Irving Ilardesty 351 



of a substance between the globules which colored differently by the 

 osmic acid. Boveri and Kupft'er thought the appearance of the network 

 to result from a stage of the decomposition of the myelin, while on the 

 other hand, Gedoelst affirmed the preformation and preexistence of the 

 network, agreeing with Pertik that it indicated the presence of two sub- 

 stances in the myelin sheath. i\Iore recently (1904) Chio, experimenting 

 with different solutions of osmic acid upon the peripheral fibers of the 

 frog and guinea-pig, reached the conclusion that the globules are a con- 

 stant form of the myelin. He finds globules present not only after the 

 action of the osmic acid but after various other reagents as well, both 

 isotonic and anisotonic. His illustrations, all of them after the action 

 of osmic acid, present the appearances usually found in imperfectly 

 blackened medullated fibers. By removing the blackened portions, the 

 globules of myelin, from his pictures, all of them may be homologized 

 with the form here indicated by A in Fig. 8 and with the gradation 

 forms between A and type B. 



Without doubt myelin exists normally in a finely divided form — an 

 emulsion. For this reason the nerves possessing it appear white by 

 reflected light. In the fresh condition, however, the individual droplets 

 are much smaller than after post-mortem exposure and treatment with 

 reagents. The larger globules, arising after death by a continual coal- 

 escence of the smaller, perhaps bear a similar relation to the condition 

 in the fresh nerve as do the globules of cream bear to those of fresh milk. 

 In the finely divided condition the myelin is distributed evenly through- 

 out the sheath framework supporting it ; in the coarser globular form the 

 normal arrangement of the framework is distorted in the characteristic 

 manner by the continued coalescence of the smaller into the larger glob- 

 ules, the conical appearances resulting from the necessary shape of the 

 large globules within the confines of the cylindrical sheath. 



In the central system there is no sheath of Schwann conforming to 

 the distinctly formed membrane investing the fibers of the peripheral 

 system. In my sections the sheath of Schwann of the peripheral fibers is 

 well differentiated by the Benda stain. It seems everywhere to be com- 

 ]:)letely separate and distinct from the framework of the sheath, as is 

 shown in Fig. 8, sch. The sheath nuclei, usually pictured as ad- 

 hering to the sheath of Schwann, by no means necessarily do so. 

 Usually surrounded by a small amount of protoplasm, they may as often 

 be found adhering to the surface of the medullary sheath as to the inner 

 surface of the sheath of Schwann. The sheath of Schwann closely invest- 

 ing the medullary sheath, the nuclei are usually in contact with both. 



The sheath of Schwann in both structure and staining properties re- 

 sembles the ordinary basement membranes of the epithelia of the body. 



