THE NEURON AND ITS CONDUCTING PATHS 117 



The same arguments may be advanced against the view that the 

 myeUn sheath serves as an insulator to prevent the overflow of an 

 impulse from one axis-cylinder to another. So far no definite proof 

 has been furnished for the contention that the non-medullated fiber 

 conducts less efficiently than the meduUated. It is frequently held, 

 however, that the loss of coordination resulting in the course of mul- 

 tiple sclerosis of the cord, is due to the destruction of the myelin sheaths 

 of these fibers, because their axis-cylinders appear to be perfectly nor- 

 mal. In general, however, it is true that the wave of excitation is 

 conducted without it spreading to neighboring fibers by contact. Iso- 

 lated conduction, therefore, is the rule. 



The third contention, that the myelin sheath serves as a nutritive 

 medium for the axis-cylinder, is based upon the following data. It 

 has been found that its thickness varies directly with the caliber of the 

 axis-cylinder, and that the axons of the projection system of the cere- 

 brum are the thickest of all. Moreover, inasmuch as staining reagents 

 find ready access to the axis cylinder through the different indentations 

 at the nodes of Ranvier, it has been supposed that the nutritive sub- 

 stances select the same course. It has also been observed that the 

 stimulation of a nerve is followed by certain structural changes in the 

 myelin sheath, consisting in a widening of the meshes of its neurokera- 

 tin framework.^ Medullated fibers are also said to be more irritable 

 and to possess greater recuperative powers than non-medullated. 

 None of these facts, however, is sufficiently definite to constitute an 

 actual proof of the aforesaid view. In addition, it might be men- 

 tioned that the axis-cylinder and the myelin sheath have really a 

 separate origin, because the former is an outgrowth from the cell- 

 body, and the latter, from the mesoblastic cells surrounding the 

 axon. This histogenetic peculiarity is also betrayed by the changes 

 which an adult nerve-fiber undergoes in the course of degeneration 

 and regeneration. The latter prove conclusively that the axis-cylinder 

 is nourished from the cell-body, while its investments derive their 

 nutritive material directly from neighboring blood-vessels. 



The neurolemma is generally regarded as a supporting and protect- 

 ing membrane and plays an important part in the degeneration and 

 regeneration of nerve tissue. The view that it is also insulating and 

 nutritive in its function could be met with the objections enumerated 

 previously. 



Degeneration of Nerve. — The nerve-fiber regains its normal appear- 

 ance and function only as long as it remains in connection with the 

 cell-body. When its continuity is broken by cutting, crushing, heat- 

 ing or other means, the fiber loses its irritability and conductivity 

 and undergoes very characteristic retrogressive changes. Directly 

 after the injury, however, its excitability is temporarily increased and 

 especially at the seat of the trauma, owing, in all probability, to the 



1 Striibel, Pfluger's Archiv, cxlix, 1912, 1. 



