ix ELECTRICAL EXCITATION OF XERVE 165 



law relate nut so much to unequal diminution of excitability at 

 different points of the nerve, as to impaired conductivity. 



On the other hand, it is a well-known and easily confirmed 

 fact that the response of a nerve to weak electrical stimuli is 

 considerably heightened by loss of water ; and that indeed as 

 was pointed out more especially by Harless and Birkner (36) 

 at a time when the spontaneous twitches which induce the so- 

 called desiccation-tetanus are still completely absent. Griin- 

 hagen and Mommsen (36) have recently shown that " a nerve is 

 the more sensitive to the effect of the electrical current in pro- 

 portion with its loss of water, especially when the characteristic 

 spontaneous twitches make their appearance." Hence it is of 

 interest to see whether the opening of a battery current of low 

 intensity acts in this case as sufficient stimulus. Harless dis- 

 covered that after partial loss of water in the nerve, the opening 

 twitch was discharged by weak ascending, as well as descending, 

 currents, and it is easy to confirm the truth of this fact by the 

 simple experiment of exposing a frog's nerve, laid over unpolar- 

 i sable electrodes at not too high (room) temperature, to gradual 

 evaporation, a.nd exciting it from time to time with ascending or 

 descending currents, the intervals not being excessive. It is 

 advisable in these experiments to use a nerve-muscle preparation 

 still connected with the spinal cord, 1 in order completely to 

 exclude the action of the cross -section, although precisely the 

 same results are obtained on exciting the peripheral tracts of 

 divided nerves. Moreover, in the last case the preparations with 

 divided nerves may be left for several hours (Mommsen's process) 

 in 0'6 ' \ NaCl, in order to equalise the alterations of excitability 

 caused by section. 



V. CLOSING AND OPENING TETANUS THROUGH ELECTRICAL 



EXCITATION 



1. Conditions of Production by Excitation of Frog's Nerve 



The first effect of commencing desiccation appears in the 

 graphic record of the muscle - contraction, as a more or less 



1 Allusion to a nerve still connected with its centre always implies a preparation 

 taken from a chloralised frog, consisting of the isolated vertebral column (after 

 destroying the brain), sciatic nerve, and corresponding gastrocnemius muscle, on the 

 same side. 



