CIRCUMSTANCES AFFECTING NERVE EXCITABILITY. 479 



some nerve cell. The severance of the fibre cuts off the distal portion 

 from such cellular connection. If this is carried out in the mammal, 

 with no appreciable disturbance of the circulation, and the animal kept 

 alive, changes manifest themselves in the distal portion (Wallerian law 

 of degeneration). The substance of the sheath in medullated nerve is 

 altered, so that this, instead of being a continuous mantle, is changed to 

 a series of discontinuous swollen masses; the axis cylinder is broken 

 up, arid the primitive sheath nuclei are multiplied. Finally, the 

 whole is invaded by wandering cells, the fragments of axis cylinder 

 and medullary sheath disappear, and the nerve fibre is replaced by a 

 strand of connective tissue. All these changes are gradually developed, 

 commencing within a few days of the section, and being completed after 

 some weeks or months. The initial change may commence within 

 forty-eight hours of the section in the mammal, and soon presents 

 itself throughout the whole length of the distal portion of the fibre. 

 The maintenance of the normal condition of a nerve fibre, both structur- 

 ally and functionally, is thus dependent upon the preservation of 

 functional continuity with the cell of which it is a process, and there 

 is reason to believe that the nutritive influence exerted by the cell 

 is associated more especially with its nucleus. A mammalian nerve 

 with unimpaired circulation, if severed from its cell connection, soon 

 loses its fundamental physiological properties. In the case of the 

 sciatic of the rabbit, stimulation below the point of severance evokes 

 little or no response in the muscles to which it is distributed. This 

 functional failure may appear forty-eight hours after the division of 

 the nerve ; it pervades the whole distal portion, and is evidently 

 associated with loss of conductivity dependent upon the breaches in 

 axis cylinder continuity, produced by degenerative changes. 



Galvanometric observations made by the writer appear to show that, after 

 three or four days, the rabbit's sciatic still gives indications of a localised 

 electrical response, although no transmitted one can be detected. 



The time of onset of the failure of function in consequence of degene- 

 ration differs very greatly, not only in different mammals, but in different 

 nerves of the same animal. Thus Arloing found that with correspond- 

 ing motor nerves of the dog and monkey, the section in the former caused 

 total loss of functional conductivity in four days, whereas in the latter 

 this failure was not established until a longer period. The time of 

 onset of the failure differs in the sciatic, median, facial, and vagus of 

 the same monkey. The different nerve fibres of any one nerve trunk 

 vary in this respect ; the well-known experiment as to vaso-constrictor 

 and vaso-dilator fibres in the sciatic of the frog is an illustration of this, 

 the latter maintaining their excitability longer than the former. The 

 different fibres of the divided vagus nerve of the monkey lose their 

 excitability at different times, the cardio-inhibitory fibres in eight days, 

 whilst the motor fibres for the larynx and oesophagus may be still 

 excitable after thirteen days. 1 



The regeneration of medullated nerve appears, from the work of 

 Eamier, which has been recently confirmed by Stroebe, 2 to be largely 

 due to the growth of new medullated fibres from the central ends of 



1 Arloing, Arch, de physiol. norm, et path., Paris, 1896, p. 75 ; see also Langley and 

 Anderson, Journ. Physiol. , Cambridge and London, 1896, vol. xix. p. 381. 



2 Beilr. z. path. Anat. u. z. ally. Path., Jena, 1893, Bd. xiii. S. 160. 



