438 THE PROPERTIES OF STRIPED MUSCLE. 



But this union is not the only, or even the most important way in which 

 the physiological action of compression manifests itself. A muscle of 

 which one end is squeezed, passes in consequence into such a state of 

 abnormal excitability, that a stimulus which would ordinarily evoke a 

 single twitch is followed by a persistent tetanus-like spasm. So long 

 as this spasm lasts, the muscle acts on the physiological rheoscope with 

 the greatest energy, producing secondary contractions which, when 

 they are recorded graphically, seem to consist of an arhythmical series 

 of unequal twitches. Kiihne explains this effect as produced by a kind 

 of electrical storm in the compressed muscle, and attributes to the 

 intensity of the electromotive changes of which it is the seat so long as 

 the spasm lasts, the extraordinary energy with which it acts on the 

 rheoscopic limb. 1 



A state of altered excitability similar to that which is produced by 

 compression, manifests itself in muscles which have been subjected to 

 partial dessication, e.g. in the limbs of frogs, which, after having been 

 stripped, are left exposed. In limbs so prepared, slight mechanical 

 irritation produces spasm which spreads not only to the whole of the 

 muscle irritated, but to its neighbours. Biedermann has also found that 

 if two muscles, of which the surfaces have been allowed to dry, are 

 applied without pressure to each other, the spasm of the muscle 

 primarily excited is faithfully imitated by the other ; and, further, that 

 muscles so acted on have the same power of exciting secondary neuro- 

 muscular spasm in the rheoscopic limb as compressed muscles. The 

 analogy thus indicated between the two effects has led both Biedermann 

 and Ktihne to believe that the abnormal excitability is attributable in 

 the two cases to the same cause privation of water. In the one case 

 water is squeezed out of the compressed part, which thereby becomes 

 in the excited muscle a focus from which irregular waves of excitation 

 emanate; in the other, the surface fibres which have lost water by 

 evaporation play the same part. 



Voltaic currents of brief duration Induction currents. 

 When a current is closed for a very small fraction of a second, e.g. 

 Ttnj" through a curarised muscle, those characters of the response 

 which are dependent on its duration disappear. The effect produced is that 

 of a single instantaneous excitation, the seat of which is at the cathode. 

 An excitation wave, with its accompanying contraction wave, starts 

 from the cathode in either direction. If the current is of sufficient 

 strength, a closure of - 005 seconds evokes a maximal response. If 

 the closure is shortened, the height of the resulting twitch diminishes 

 with the duration. This can be best shown by a method devised by 

 Fick many years ago. The instrument 2 used for the purpose is such 

 that a brass tongue, springing from a revolving horizontal bar, rubs 

 against an insulated triangular brass plate in its revolution, the brass 

 plate and the tongue being in circuit. The time of contact can be 

 varied (1) by varying the rate of rotation, and (2) by adjusting 

 the plate so that the tongue rubs over a greater or less extent of its 

 surface. Experiments made on Fick's plan show that even when the 

 closure is so short that the anodic excitation is indistinguishable, the 

 excitatory process may yet be of a measurable duration. 



1 Loc. cit., Bd. xxvi. S. 222. 



2 A full description of the instrument will be found in Tick's " Medizinische Physik." 

 1866, S. 425. 



