x ELECTROMOTIVE ACTION IN NERVE 231 



when divided in the living animal, exhibited a normal current 

 weeks and months afterwards, although excitability disappeared 

 8-14 days after the operation. Schiff asserts that the axis- 

 cylinder was still present, in spite of advanced disintegration of 

 the medullary sheath, a further proof of the physiological signifi- 

 cance of this portion of the fibre. 



The alterations in time suffered by the demarcation current 

 in medullatecl nerve are of extreme interest, since they are 

 analogous with those of cardiac and smooth muscle. Engelmann 

 (9) found that the E.M.F. of the cross-section fell with extreme 

 rapidity, and appeared again in undiminished vigour when a 

 new section was made. This he explained from the fact that 

 the individual cells die separately, notwithstanding their physio- 

 logical coherence the process of death is confined to the cells 

 that are directly injured. Similar relations appear in medullated 

 nerve-fibres, although these cannot be regarded as consisting of 

 separate cells fused together. After only 1-2 hours Engelmann 

 noted that the E.M.F. of the artificial cross-section fell from 

 60 to 25 per cent of the initial value, in 2024 hours to at least 

 35'5 per cent, more often to ; frequently, as pointed out above, 

 there was a weak reversed current. Eenewal of the cross-section 

 in every case restored the full value of the original current. 



Head (10) found, on repeating Engelmann's experiment, that 

 the diminution of the demarcation current was especially marked 

 in the nerves of summer frogs, so that the increase of E.M.F. in 

 consequence of the new cross-section is here particularly striking. 

 After 14 minutes the very pronounced current of rest was 

 observed by Head to fall to 1 of its original value. Twenty-eight 

 minutes after beginning the experiment a new section was made, 

 upon which the nerve current at once reappeared in its former 

 vigour. As a rule, there was even a marked rise of E.M.F. 



O 



beyond the original magnitude. In one special case the current 

 of rest in a frog's sciatic gave a deflection of 155 degrees, which 

 fell 20 minutes later to 32 degrees of the scale. After making 

 a new section, the current at once increased to 120 degrees, and, 

 after a second rapid fall, gave a deflection of 232 (!) degrees, on 

 applying another (fourth) cross-section, 33 minutes after beginning 

 the experiment. Engelmann explains this striking reaction from 

 the fact that the process of mortification in the injured fibres is 

 arrested at the nearest node of Eanvier. The same effect appears, 



