426 ELECTRO-PHYSIOLOGY CHAP. 



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 



