SECONDARY MUSCULAR ELECTRICAL EXCITATION. 437 



through its nerve at intervals, say of \ second, the secondary twitches 

 are exact counterparts of the primary ; and if the frequency is sufficiently 

 increased, tetanic contractions are obtained which cannot be dis- 

 tinguished from primary ones. When the same experiment is made 

 after the primary muscle has become exhausted, it often happens that 

 the secondary muscle responds to tetanus of the primary by a single 

 initial twitch, in which case the normal result may be expected to 

 reappear if the muscle is allowed to rest. 



It is not by any means essential that the primary excitation should 

 be indirect. A curarised sartorius, whether excited by a single induc- 

 tion current, or tetanised in the same way, evokes corresponding neuro- 

 muscular responses ; but this effect, namely, the secondary twitch in 

 response, to direct excitation of the primary muscle, is best obtained 

 when a battery current is led for a few millimetres along the fibres 

 of the uninjured muscle to a terminal cathode. 1 



The secondary neuro-muscular response has long been recognised 

 as a means of distinguishing different kinds of continuous contraction. 

 Its value for this purpose can be best judged of by referring to the 

 instances in which it fails : 



1. No continuous contraction of central origin, whether cerebral or 

 spinal, has as yet been observed to evoke a secondary neuro-muscular 

 response of the same character as the primary, with the striking excep- 

 tion that the reflex muscular contractions of frogs poisoned by strychnine 

 induce in the physiological rheoscope neuro-muscular contractions which 

 have a strong superficial resemblance to tetanus. When they are 

 recorded graphically, they are seen to consist of a series of twitches, 

 following each other with a frequency of about eight or ten per second, 

 each of which corresponds to the beginning of one of the series of 

 spasms which constitute the primary response. 2 



2. Secondary neuro-muscular spasm is, as a rule, not produced by any 

 of the prolonged contractions which are evoked by continuous stimulation, 

 whether direct or indirect. As examples may be mentioned the closing 

 continuous contraction by indirect excitation (Bitter's tetanus) ; 3 the 

 spasm consequent on partial drying of a nerve ; or that produced by 

 the action of common salt thereon. 4 There is, however, an exception to 

 this general statement of such importance that it must be specially 

 considered. 



Some years ago, the fact that a muscle can be excited directly by its 

 own current, or by that of a beating heart of the tortoise, suggested to 

 Kiihne the inquiry whether an excited muscle could be made to com- 

 municate its state of excitation directly to another muscle at rest, if 

 the two were brought into relation with each other in a suitable 

 manner. The result of this investigation was the discovery that, 

 although the mere contact of an excited muscle with an unexcited 

 produces no effect whatever in the latter, it is different if contact is 

 accompanied by pressure, and particularly that if the ends of two 

 sartorius muscles are squeezed together in such a degree as not seriously 

 to injure the compressed parts, the two muscles so " welded " together 

 respond to the excitation of either as if they were structurally united. 



1 Biedermann, loc. cit. 



2 Kiihne, Untersuch. a. d. physioL Inst. d. Univ. Heidelberg, 1880, Bd. iii. S. 59. 



3 Friedrich, Sitzungsb. d. k. AJcad. d. Wissensck.. Wien, 1875, Bd. Ixxii. Abth. 3, 

 S. 413. * Kiihne, S. 61. 



