i GENERAL PHYSIOLOGY OF MUSCLE 77 



scale during the tetanus. This is the " negative variation " referred 

 to above. On a sensitive galvanometer it can be shown during 

 single twitches as well as in tetanus. If the current of rest is 

 compensated, and the nerve is then excited, the negative variation 

 will appear on the galvanometer as an autonomous current, in 

 the opposite direction to the current of rest showing that the 

 e.m.f. of the muscle is diminished by excitation (du Bois-Pieyrnond). 



The phenomena of secondary contraction, or induced contraction 

 as it was termed by Matteucci, and secondary tetanus, which can 

 be seen in a frog's leg when its nerves are laid across the muscles 

 of another leg, so that the muscle current produced in the latter 

 on contraction passes through the nerves of the former (Fig. 42), 

 depend, as du Bois-Eeymond showed, on the exciting action of 

 the negative variation of the current. The secondary twitch is 

 the simplest and most convincing proof that a single contraction 

 can elicit a negative variation of sufficient intensity to stimulate 

 the nerve. Secondary tetanus further shows that the negative 

 variation of the primary tetanus is a discontinuous process, 

 although the variations in the current are too rapid to be 

 followed by the galvanometer needle, and their mean value only 

 is recorded. 



The oscillating character of the muscle current in tetanus can 

 also be demonstrated by the telephone, which Hermann regards 

 as more sensitive than the " galvanoscopic leg." When the 

 muscle current is led off to a telephone, a sound is heard during 

 tetanisation which results, as Bernstein and Wedensky demon- 

 strated, from a number of vibrations equal to the rate of the break 

 or make shocks of the tetanising current. 



Bernstein was able by an ingenious apparatus known as the 

 differential rheotome to analyse the negative variation during a 

 simple contraction. 



The negative variation in a nerve-muscle preparation during 

 tetanus can be photographed by reflecting a beam of light from 

 the galvanometer magnet on to a sensitive surface moving by 

 clockwork. Fig. 53 records the tetanic contraction and accom- 

 panying negative variation. 



The galvanometer does not react quickly enough to show the 

 oscillations that accompany tetanus, but if the capillary electro- 

 meter is used, they can be photographed by letting the shadow 

 of the meniscus fall on a slit behind the sensitive paper, which 

 travels in a direction vertical to the oscillations of the mercury 

 (Burdon-Sanderson and others). 



The negative variation increases to a maximum with the 

 intensity of the tetanising current. According to Bernstein it 

 never reaches the zero point, i.e. never cancels the demarcation 

 current. According to Gotch and Sanderson, on the contrary, 

 the negative variation may pass beyond the zero point, and 



