400 BIOLOGY. [Book vi. 



The persistent vitality of the fibrillary nervous tissue is not 

 less remarkable. 



A nerve exposed to the free air dries up, and soon loses its 

 properties, but, within a certain time, it may recover them by 

 simple imbibition, in the same manner that certain desiccated 

 infusoria revive. 



If we cut the anterior rachidian or motory roots of a mixed 

 nerve, the peripheric end will not be long in losing its motricity, 

 and it gradually wastes even as far as its finest terminal ramifi- 

 cations. If we cut the posterior or sensitive root, _ above the 

 ganglion, it is the central end which wastes. 



Under such conditions, the nervous fibres lose their excita- 

 bility, in the mammifers, at the end of about four days, whilst 

 the irritability of the muscles lasts nearly three months. The 

 .structure of the nervous fibre is therefore altered, but not 

 fundamentally. In efEect, the nerve has only lost its oily en- 

 "velopmentj but its axile cord lasts and persists, apparently 

 intact, covered only by the Schwann's sheath, shrivelled and 

 folded round it. Professor Schiff has found the axile filament, 

 still intact, five months after the section of a nerve ; M. Vul- 

 pian has seen the same thing after six months. But things do 

 not remain in this state. At the end of an indefinite time, 

 shorter if the animal is young, longer if the animal is old, or of 

 variable temperature, or cold-blooded, the nerve is restored, the 

 anatomical and physiological continuity is re-established between 

 the trongons of the cut nerve, if they have been kept in contact ; it 

 is sometimes re-established even when there has been a resection 

 of a nervous segment of from 1 to 2 centimetres. In this last case, 

 we see a bundle of grey fibrils, budding, shooting at the central 

 end, and joining at the peripheric end. At the same time, 

 myeline is reproduced in the Schwann's sheaths, and, finally, the 

 entire nerve is restored ; for a long time, however, its fibres have 

 a very small diameter. ^ 



This property of restoration is constant, inherent in the 

 1 SchifF, Vulpian, PWlippeaux. 



