DEGENERATION OF NERVE 



797 



phery, and more rapidly in warm- than in cold-blooded animals. At 

 any rate, that is the interpretation generally given to the fact that at 

 a given period after section the changes especially the breaking- 

 up of the myelin are more pronounced near the proximal end of 

 the peripheral stump. In a mammal degeneration is far advanced 

 in a fortnight, although the last remnants of the myelin may not 

 be absorbed for months. In the degenerated nerve (cat's sciatic) 

 the percentage of phosphorus undergoes a diminution from about 

 the third day. About the eighth day the loss of phosphorus i.e., 

 of the phosphatides (lecithin, kephalin) is markedly accelerated, 

 coinciding with the appearance of a strong Marchi* staining re- 

 action. By the twenty-ninth day the degenerated nerve is prac- 

 tically devoid of phosphorus. A progressive increase in the water 

 and a diminution in the total 

 solids also culminate about the 

 same time (Mott and Halli- 

 burton). 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 peripheral, but not on 

 the central side of the lesion, f 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. 



* The chief constituents of Marchi's solution are potassium bichromate and 

 osmic acid. It stains medullated nerve-fibres black in the earlier stages of 

 degeneration. 



| 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 'recurrent fibres,' 

 whose course is described on p. 892. 



Fig. 278. Degeneration of Spinal Nerves 

 and their Roots after Section. The 

 shading shows the degenerated portions. 



