426 ELECTRO-PHYSIOLOGY 



superposed nerve. We have in fact observed that the light 

 fibres are much more quickly fatigued than the dark fibres. 



With regard to the last point also, it can hardly be supposed 

 that the wave of variation produced at one end of a muscle 

 with parallel fibres, reaches every fibre at the same phase, and in 

 this we ought to find an explanation, not merely of the vigorous 

 excitation experienced by a nerve laid at right angles across a 

 strong bundle of such fibres, but, still more, of the otherwise 

 hardly intelligible secondary activity of the rectangular cross- 

 section. 



It is remarkable that during life the contracting muscles 

 apparently exert no secondary action upon the nerves lying 

 between them. Hering showed indeed that the twitches of 

 the diaphragm (cat) first observed by Schiff and not explained 

 subsequently, which are isochronous with the beat of the heart, 

 are produced by the contact of the phrenic nerve with the 

 beating heart. No other instance is known, and it is easier 

 to demonstrate that under the most favourable conditions, no 

 secondary excitation of extra-muscular nerves in situ results 

 from muscles foreign to them. If the sciatic nerve is cut 

 close below the departure of the branches to the thigh, the 

 muscles of the leg and foot are quiescent, even with strong 

 tetanising excitation of the same plexus, although the nerve to 

 the leg is embedded between the much-contracted thigh muscles 

 (Kiihne). It is easy to show that this cannot be referred to the 

 short-circuiting of the action current within the surrounding 

 mass of muscle. Kiihne always obtained secondary action when 

 he packed the nerve of a frog's leg in the thigh, after removing 

 the bone, and then excited the sciatic plexus, and it is well known 

 how little other moist bodies, serving as a deriving circuit, are 

 able to hinder secondary action. Thick layers of filter-paper, or 

 packing the primary muscle and secondary nerve on all sides in 

 the viscera of a female frog, produce no disturbance of secondary 

 excitation effects. That in secondary inexcitability of the nerves 

 in situ there is "a special adjustment of the muscular and 

 nervous activity, which really accomplishes much more than is 

 demanded by the natural conditions," seems evident from the fact 

 detected by Kiihne, that even a slight dislocation of the nerves 

 lying between the muscles of the thigh, or their simple exposure, 

 suffices to call out the absent secondary effect, while on closing 



