INDEPENDENT MUSCULAR EXCITABILITY 79 



the contrary, is quite as excitable to stimuli in a direction transverse 

 to the muscle fibres as to one parallel to them. Nerve again is 

 especially sensitive to currents of very short duration, whereas muscle 

 will not respond unless the duration be sufficiently prolonged. A 

 curarised muscle is much less sensitive to shocks of short duration 

 than a non-curarised one. 



In a further direction Kuhne showed that chemical stimuli which 

 were particularly irritant to muscle (such as NH 3 or very weak HC1) 

 were equally effective to both curarised and non-curarised muscles. 

 If a strong constant current be sent through a nerve, it is found that 

 the excitability of the nerve in the part immediately surrounding the 

 anode is very greatly diminished. Kuhne utilised this to lower the 

 excitability of the nerve fibres in a sartorius by throwing a constant 

 current into its nerve, placing the anode close to the muscle. A muscle 

 thus treated was found to be just as excitable to ammonia or weak 

 hydrochloric acid, whilst those forms of stimuli, such as glycerine, 

 which act on nerve only, are now without effect, or only produce one 

 when the excitation becomes excessive. 



If the nerve supplying a muscle be cut and allowed to degenerate 

 for some days, the response of the muscle to electrical stimuli becomes 

 considerably modified ; while the intra-muscular nerve endings are 

 intact the muscle responds more readily to induced shocks than to 

 the constant current, whereas when these terminals have degenerated 

 the reverse is found to be the case. This change of condition is 

 explicable on the fact already tested in experiment 2, which shows 

 that muscle is much less excitable to currents of short duration than 

 nerve. 



EXCITATION OF MUSCLE BY A CONSTANT CURRENT 



In our experiments up to this point, we have as a rule employed 

 an induced shock whenever we wished to stimulate a muscle or its 

 nerve, but we have also seen that a muscle is excitable to thermal, 

 mechanical, and chemical stimuli as well as to electrical. We have 

 now to consider the response of muscle to electrical stimuli other than 

 induced currents. We found that muscle was less excitable to induced 

 currents than nerve, and this is found to be due to the very short 

 duration of these currents. 



If a constant current of sufficient strength be sent through a muscle 

 a contraction occurs at the instant of make of the current and again 

 at the instant of break, but no effect is as a rule produced during the 

 passage of the current. These two contractions are different, not only 



