118 PHYSIOLOGY OF MUSCLES AND NERVES. 



protected from direct contact with each other by gutta- 

 percha or by some other substance. The comparison 

 is, however, but superficial. No electrically-isolating 

 membrane can really be discovered in any part of the 

 nerve-fibre, but all their parts conduct electricity. 

 When, as we shall presently find, electric processes 

 occur within the nerve, these standing in definite re- 

 lation to the activity of the nerves, we must assume that 

 isolation as it occurs in the nerves is not the same as 

 in telegraph wires. We cannot here trace the matter 

 further, but must accept the fact of isolated conduction 

 as such, reserving its explanation for a future occasion. 

 5. On irritating the nerves by means of currents 

 from an inductive apparatus, it is found that the pulsa- 

 tions which occur are sometimes strong, sometimes 

 weak. All nerves are not alike in this respect, and 

 even the parts of one and the same nerve are often 

 very different. We must accordingly suppose that 

 nerves are variable in the degree in which they receive 

 irritation. This is spoken of as the excitability of the 

 nerve, to express the greater or less ease with which 

 they may be put in action by external irritation. Two 

 ways may be adopted to measure the excitability of a 

 nerve or of a certain point in a nerve. Either the 

 same irritant may always be used, and the excitability 

 may be determined by the strength of the muscular 

 pulsation evoked by this irritant; or the irritant may 

 be altered until it just suffices to evoke a muscular 

 pulsation of a definite strength. In the former case 

 it is evident that the excitability must be estimated 

 as higher in proportion as the muscular pulsation pro- 

 duced by the irritant is stronger; in the latter case 

 the excitability is said to be greater in proportion 



