Mammalian Nerve- Muscle and Rejiex Preparations. 343 



were fixed and the contraction of tlie muscle was directly recorded. Other- 

 wise the method was similar to that already described for the extensor reflex 

 preparation. 



I. — Experiments on Nerve-Muscle Preparations. 



The threshold value of stimulation for this preparation was very low as 

 compared with that in reflex preparations. One or even half a division of the 

 rheochord wire, through which a constant galvanic current flowed fed by one 

 accumulator, usually suffices to provoke an appreciable contraction at closure 

 of either descending or ascending current. With a fresh motor nerve the 

 contraction at make generally appears with weaker ascending than descending 

 currents. With currents of moderate strength in either direction excitation 

 occurs both at make and break. This second stage of the "excitation 

 formula " (Pfliiger's law) appears with a rather weak current ; two divisions 

 of the rheochord are usually enough to induce opening contractions. 



Very strong currents are, however, necessary to demonstrate the third 

 stage of this formula. As a rule, I obtained no excitation at make of the 

 ascending current, using the full potential difference obtainable from three 

 accumulators (6 volts). It was, however, difficult to decide whether or not 

 there was a contraction at make of the descending current, since the muscle 

 was thrown into tetanus during its passage and appeared to relax without any 

 further contraction when the flow of the current ceased (fig. 1). There is no 

 doubt, however, that there is no contraction at break of a strong descending 

 current in preparations where the motor nerve has been divided forty-eight 

 hours before the experiment, as the muscle responds only at its make, 

 remaining quiescent all the time the current is passing. 



The height and form of the closure- and opening-contractions are different 

 according to the strength and direction of the current. Though these 

 contractions are not twitches in a strict sense, they are twitch-like if produced 

 by weak currents. The contractions at make of the descending and at break 

 of the ascending current increase with the intensity of the stimulation, while 

 those at break of the former and at make of the latter attain their maximum, 

 remaining on it within a considerable range, then diminish their height with 

 further strengthening of stimulation, and finally fail to appear when the 

 current is still stronger (fig. 1). 



With weaker polarising currents there is po continuous excitation during 

 their flow through the nerve in either direction. With progressive increase 

 of the strength of the descending current, the make-contraction is no longer 

 twitch-like, but is a brief tetanus, subsiding somewhat slowly. The stronger 

 the current the more prolonged is the contraction, until such strength is 



VOL. CXII. — B. 2 D 



