42 
10. — Tetanus and Fatigue. 
When a second momentary current is sent into a muscle 
before the contraction due to the first has subsided, the effect 
of the second current is added to that of the first, and a new- 
contraction appears superimposed upon the old, starting from 
the degree of contraction at which the latter had arrived, 
and proceeding much as if that were the normal condition of 
the muscle ; with succeeding currents the process goes on 
until a certain limit of contraction is reached, beyond which 
the muscle cannot go. If the shocks follow one another 
quickly enough the recording lever will trace upon a travel- 
ling surface a straight line, and the muscle is said to be in 
tetanus, and it will, if the shocks are kept up, continue in 
this condition until ''fatigue" sets in, and the lever gradually 
sinks. 
Helmholtz considered that ''from the point at which Ire 
second excitation becomes effective the twitch behaves as if 
the contracted state of the muscle at the moment was its 
natural state, and the second twitch, alone, induced m »t " 
It has been found, however, that this is not true even for the 
second twitch ; it is lower than the first and of a shorter 
period,^' while it is obviously not applicable to the later 
twitches when the limit is nearly reached. The reason for 
this summation is, of course, the repeated discharge of ions 
from the seat of stimulation -the twitches will become smaller 
and smaller and shorter as the ion-proteid is used up — and no 
increase of contraction can then take place. At this period, 
however, since a great mass of kations have been rapidly 
liberated, they cannot diffuse at once into the spongioplasm 
so as to diminish the difference of potential at the contact sur- 
face ; so that the muscle remains for some time in tetanus, 
and only as the kations diffuse into the spongioplasm will the 
lever sink and the muscle enter into "'fatigue"— finally the 
lever sinks quite, and the muscle is isoelectric — or, only with 
the usual contact difference of potential between its hyalo- 
plasmic and spongioplasmic surfaces. An objection may be 
raised : Why do rapidly succeeding shocks produce reiterated 
contractions when a constant current fails to cause persistent 
contraction ? There are two reasons : First, that to produce 
complete tetanus in striated muscle the shocks must be of 
extremely short duration ; and we have seen that such shocks 
do not discharge so many ions as longer ones : that is, there 
is a reserve left, while the muscles in which the shocks aeed 
not be so short are just those in which persistent closure con- 
* Biede^miann : Electro-physiology: Trans, by F. A. Welby, 
vol. i., page 115. 
