THE NEURON AND ITS CONDUCTING PATHS 121 



of the neuron. As far as the dendrites and axons are concerned, it is 

 conceivable that they are nourished by neuroplasmic streams from the 

 cell-body, whereas the nutritive supply of the investments is derived 

 from neighboring blood-vessels and lymphatic channels. The metab- 

 olism of both, however, depends upon the functional capacity of the 

 neuron as a whole. The cell-body, therefore, constitutes the trophic 

 center of the neuron and the element chiefly concerned in this func- 

 tion is the nuclear material. This deduction may be justified by the 

 analogy that the survival of a cell depends upon the preservation of 

 its nuclear substance. Thus, if a cell is divided several times, its 

 different fragments must soon disintegrate, unless a sufficiently larg:e 

 mass of the nucleus have been apportioned to each of them. 



In describing the histological alterations occurring in a disin- 

 tegrating neuron, attention should first be called to the degeneration 

 involving the fiber separated from the cell-body and secondly, to the 

 retrogressive changes affecting the cell-body and its dendrites. Con- 

 cerning the former it should be noted that the primary degeneration 

 remains confined to the seat of the injury and advances only as far 

 as the second or third node centrally and distally to it. The stretch 

 of fiber so affected measures no more than 3.0 mm. in length. From 

 here this process spreads so rapidly that it becomes practically 

 simultaneous throughout the distal stump. ^ A typical Wallerian 

 degeneration is initiated by a loss of irritability which is associated 

 with a turgescence and a fiagmentation of the axis-cylinders. These 

 changes develop two or three days after the injury.^ They are quickly 

 followed during the next day by a fragmentation of the myelin sheath. 

 The latter breaks up into ellipsoidal segments and then into smaller 

 drops, each of which contains a short stretch of the axis-cylinder 

 appearing as a complex of colorless granules. Naturally, these struc- 

 tural alterations of the myelin substance are associated with certain 

 chemical changes which betray themselves by its different staining 

 qualities.^ This particular phase of the degeneration is followed by 

 a period during which much of the material thus formed is gradually 

 absorbed so that at the end of one month the fiber is practically with- 

 out its medulla. Meanwhile, the nuclei of the neurolemma have 

 greatly increased in number and have become invested by a layer of 

 protoplasm which thus partially occupies the place of the absorbed 

 myelin. This structure is known as the " band fiber. ' * Its appearance 

 is of course very different from that of a normal nerve-fiber and there 

 is sufficient evidence at hand to prove that it is non-conductile. 



In this connection it should also be mentioned that the distal and 

 central stumps in the immediate vicinity of the lesion are frequently 

 beset with neurofibrillar outgrowths from the axis-cylinder. These 

 rami, however, cannot be considered as indications of regeneration, 



^ Ranson, Jour. Comp. Neur. and Psych., xxii, 1912, 487. 



2 Bethe and Monkeberg, Arch, fur mikr. Anat., Uv, 1899, 135. 



' Mardi's method of staining with osmium after treatment in a chrome solution. 



