96 WILLIAM TOWNSEND POETER. 



jointed chain, and removing the danger of injury from the 

 sudden bendings to which many peripheral nerves are ex- 

 posed. This can be accepted as one of the uses of peri- 

 pheral constrictions^ but the objection to Ranvier's explana- 

 tion falls to the ground with tbe confirmation of Torneaux and 

 Le Goff's discovery, and the constrictions may be looked upon 

 as a food-way to the axis- cylinder. 



Constrictions are present in the spinal cord ; the sheath of 

 Schwann is absent from the spinal cord ; therefore the sheath 

 of Schwann is not concerned in the formation of constrictions 

 in central fibres, and is probably not an essential part of peri- 

 pheral constrictions. We can safely reject the dictum of 

 Hans Schultze ■} The sheath of Schwann is " die formgebende 

 Ursache der Ranvier'schen Markunterbrechungen," an idea 

 prominent in many researches, and may look with suspicion on 

 theories that find in Schwann's sheath an explanation of the 

 structure of the constrictions. 



Further inferences are very tempting. The central nerve- 

 fibres are epiblastic, and lie in an epiblastic framework 

 (Geriist) ; no mesoblastic tissue ensheathes them (Gierke) ; 

 in Palsemon squillathe medullatednerveshaveno connective- 

 tissue sheath ; therefore, the myelin of central fibres, like the 

 axis-cylinder, is of epiblastic origin, and the medullary sheaths 

 of peripheral nerves are probably formed from the axis- 

 cylinder. If the myelin is formed from the protoplasm of 

 the axis-cylinder, then the function of the sheath of Schwann 

 is that of a simple connective-tissue envelope. The many 

 theories of development and degeneration that affirm a generic 

 relation between the sheath of Schwann and the medullary 

 cylinder are incorrect; spinal nerves are developed and regene- 

 rated through their cells of origin, and changes in the nuclei of 

 Schwann's sheath are not the cause of the growth or decay of 

 the medullary substance. But the evidence in our possession 

 does not warrant our going so far, for, although Gierke^ de- 



' ' Axen-c^lindcr und GanglioiizoUe,' p. 27. 



^ i.e. Axis-cylinder and medullary shcatli. 



' "Die Stiitzsubstanz des central Nerveusystenis," Arcli. f. niikr. Auat.,' 



