480 NERVE. 



the normal nerves. These invade the degenerated tracts, which offer 

 especially favourable media for their growth, and serve as guides to the 

 appropriate terminal structures. The suture of a divided mammalian 

 nerve, such as the sciatic of the rabbit, is followed by an outgrowth of 

 afferent fibres, whose excitation is capable of evoking reflex effects in 

 eight days; the establishment of motor connections of the efferent 

 fibres with the muscles is slower, movement not being possible until 

 about forty days. The actual time of recovery must differ both in 

 different animals and nerves. 



Erb J states that after local compression of the nerve, the restoration 

 of movement through spinal cord activity takes place at a date when 

 the nerve peripheral to the injury is still inexcitable. This result the 

 writer has failed to confirm ; if true, it is an instance of the recovery 

 of nerve conductivity for specific impulses, when excitability to external 

 stimulation is still unrestored, and furnishes another instance of the 

 separation of these two aspects of nerve activity. 



Excitability and conductivity in different nerves, and in different 

 parts of the same nerve. The comparative excitability of different 

 nerves may be ascertained by allowing the same stimulating electrical 

 current to traverse simultaneously different nerves ; or the bundles of 

 different nerve fibres, which are bound up by epineurium into one nerve 

 trunk. A well-known observation is that first made by Bitter, and con- 

 firmed by Eollet, 2 as to the minimal intensity of nerve stimulus adequate 

 to arouse responses in the groups of muscles supplied respectively by the 

 peroneal and tibial branches of the sciatic nerve of the frog. If the 

 whole trunk is excited, the first-named group of muscles contract with 

 an intensity of electrical stimulus which is inadequate to evoke a 

 response in the second group; hence the neuromuscular apparatus of 

 the flexors and abductors is more excitable than that of the extensors 

 and adductors, and on etherisation a stage may be reached when ex- 

 citation of the sciatic causes contraction of flexors only. It does not 

 necessarily follow from these and other analogous experiments, that the 

 nerves of the flexors possess greater excitability, since the whole neuro- 

 muscular connection is involved in the production of the effect. Thus 

 Albanese has shown that a similar result is obtained after simul- 

 taneous local impairment of function in the peripheral neuro-muscular 

 mechanism of both flexors and extensors. 3 The difference between 

 flexor and extensor response obtains for the rabbit as for the frog and 

 toad, and for mechanical and chemical as well as electrical stimuli. 4 

 Differences have been also observed in the neuromuscular mechanism 

 of the larynx, the adductors responding to a weaker stimulus of the 

 laryngeal branch of the vagus than that necessary to evoke a response 

 in the abductors ; and as regards the normal response to central nervous 

 discharge, the abductors fail first through the local action of ether, 

 cold, etc. 5 



A striking example of the varying excitability of the different nerve- 

 fibres composing a nerve is furnished by the remarkable response of the 

 crayfish muscles. The excitation of the nerve, with stimulus of 



1 Deutsches Arch. f. klin. Med. , Leipzig, 1869, Bd. v. 



2 Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1876, Bd. vi. Abth. 3. 



3 Arch.f. exper. Path. u. PharmakoL, Leipzig, 1894, Bd. xxxiv. 



4 Arch.f. d. ges. PhysioL, Bonn, 1868, Bd. i. 



5 Horsley and Semon, Brit. Med. Journ., London, 1886, vol. ii. 



