NERVE 607 



mammal it is far advanced in a fortnight, although the last 

 remnants of the myelin may not be absorbed for months. 



In the degenerated nerve the substances soluble in ether 

 are relatively increased, owing in part to fatty degeneration 

 of the axis-cylinder. The percentage of phosphorus is 

 markedly diminished (Mott and Barratt). In the portion 

 of the nerve-fibre still connected with the nerve-cell the 

 degeneration only extends as far back as the next node of 

 Ranvier, and seems to be due to the direct effect of the 

 injury. In non-medullated fibres, such as the fibres arising 

 from the cells of the superior cervical ganglion (Tuckett) 

 the degeneration is confined to the axis-cylinders. It begins 

 in about twenty-four hours after section, and the loss of 

 excitability and conductivity is complete by the fortieth 

 hour. 



It follows from what has been said as to the position of the 

 cells of origin of the root fibres of the spinal nerves that 

 section of the anterior root causes degeneration on the peri- 

 pheral, but not on the central side of the lesion.* Only the 

 anterior root fibres in the mixed nerve degenerate. 



Section of the posterior root above the ganglion causes 

 degeneration of the central stump, but not of the portion 

 still connected with the ganglion, nor of the posterior root 

 fibres below the ganglion or in the mixed nerve. 



Section of the posterior root below the ganglion causes 

 degeneration of the fibres of the root below the section and 

 in the mixed. nerve, but not above it. 



Degeneration of the nerve is followed, if its divided ends 

 are not kept artificially apart, by a process of regeneration, 

 already distinct under favourable conditions in from three 

 to four weeks after the section, and indeed in some cases 

 commencing as early as the second week. This consists in 

 the outgrowth of new axis cylinders, in the form of fine 

 fibres, from the ends of the divided axis cylinders of the 

 central stump of the nerve. These push their way into and 

 along the degenerated fibres, ultimately acquire a medullary 



* A few fibres in the peripheral stump of the anterior root do not 

 degenerate, and a few fibres in the central stump do. These are the 

 4 recurrent fibres,' whose course is described on p. 699. 



