308 PHYSIOLOGY 



the first stimulus has reached its culminating- point. If the first 

 stimulus was maximal it is evident that no further addition to the 

 molecular change could occur as a result of the incidence of the second 

 stimulus. 



THE EFFECT OF TEMPERATURE ON EXCITABILITY. It was 

 found by Gotch that the excitability of a nerve within certain 

 limits was increased by cooling the nerve and diminished by raising 

 its temperature (Fig. 122). Thus, if a frog be cooled to 2 C. or 3 C. 

 for a day, it will be found that simple section of the sciatic nerve 

 may suffice to send the gastrocnemius into continued contraction, 

 and under these circumstances ' closing tetanus ' may be obtained 

 with the greatest ease. This increase of excitability does not apply 

 to all kinds of stimuli. In the case of nerve its irritability was 

 found to be increased by warming, and diminished by cooling for 



FIG. 122. Tracing of muscle contractions to show effect of cooling a nerve 

 on its excitability. The lower line indicates the changes in temperature 

 of the excited part of the nerve. The muscle responded only when the 

 nerve was cooled, the stimulus becoming ineffectual when the nerve 

 was warmed. ( GOTCH.) 



induction shocks and for all galvanic currents of less duration than 

 005 sec. In skeletal muscle Gotch found the excitability for all 

 forms of stimuli increased by cooling. Lucas has shown that 

 these paradoxical effects in nerve, namely, increase of excitability 

 towards currents of long duration and the simultaneous decrease 

 towards currents of short duration, are conditioned by two opposed 

 changes in the tissue. The fall of temperature delays the subsidence 

 of the excitatory process, but at the same time renders more difficult 

 the initiation of a propagated disturbance. The first of these effects 

 reduces the current required for excitation in a ratio which is greater 

 the greater the duration of the current. The latter increases the current 

 required in the same ratio for all durations. If then the change of 

 temperature is such that the two opposite effects are exactly balanced 

 at a certain medium duration of current, it follows that for currents 



