Mammalian Nerve-Muscle and Rejiex Preparations. 



347 



In some flexor preparations, both decerebrate and spinal, with freshly 

 isolated nerves, no continuous excitation takes place during the passage of 

 either fairly strong ascending or descending currents. More usually, however, 

 reflex tetanus is produced at make of the current, if the current be strong and 

 of descending direction. The intensity of current necessary to produce this 

 result is far greater in these preparations than in the nerve-muscle prepara- 

 tion. Moreover, this closure reflex tetanus usually subsides some time during 

 the course of prolonged stimulation. 



With the flexor reflex preparation therefore, even when that preparation is 

 of the spinal kind, which is more readily responsive reflexly than the 

 decerebrate, the galvanic current even when strong evokes from the afferent 

 nerve a contraction only at make and break, and not during the maintained 

 passage of the current, or does so only exceptionally. This is a striking 

 difference from the nerve-muscle preparation. 



III. — Experiments on the Extensor Keflex Preparation. 



The experimental results in this preparation are more complex, and it is 

 sometimes difl&cult to analyse the various factors concerned in causing this 

 complexity. Under favourable conditions the results, though manifested in a 

 different way, agree in regard to the effects of changes in strength and 

 direction of current with those obtained in the nerve-muscle and flexor reflex 

 preparations. These reactions can be more easily understood when the 

 influence of certain characteristic features of the central nervous system is 

 considered. Weaker or less abrupt stimulation of an ipsilateral nerve, as 

 Sherrington and Sowton (12) showed, tends to produce contraction in the 

 knee-extensor, while stronger or more abrupt stimulation always results in 

 inhibitory relaxation. Constant currents are more suitable for the study of 

 this phenomenon than induction shocks. The other important features of the 

 central nervous mechanism which are related to the present enquiry are tonic 

 plasticity (10) and the " rebound " phenomenon. 



The general experimental results with this preparation are recorded in four 

 sections. 



(1) The Validity oj P finger's Law. 

 In excitable preparations, the reflex movement, whether excitatory or 

 inhibitory, appears with smaller intensity of stimulation at make of the either 

 descending or ascending current than at its break. If, however, the threshold 

 value is rather high from the first the reflex effect at break of the ascending 

 current has a threshold which ^is either lower than or equal to that at its 

 make (see above). Even in this case it is always greater at make of the 



