ELECTRICAL STIMULATION 307 



may together produce an excitation. In the case of striated muscles ; 

 in order that mechanical summation of contraction may take 

 place, the second stimulus must become effective before the muscle 

 has completely relaxed ; the second contraction, that is to say, 

 starts from the height to which the first contraction has brought 

 the muscle. A similar condition of things appears to hold for summa- 

 tion of stimuli, if we substitute for mechanical change in muscle the 

 molecular change which accompanies the excitatory state. For 

 summation of two stimuli to take place, the second stimulus must 

 occur at a time before the condition excited by the first stimulus has 

 passed away. The maximum time at which summation of two 

 stimuli can take place will therefore vary from tissue to tissue^and 

 will bear a relation to what we have designated the ' excitation time ' 

 of the tissue and also to the rapidity of current gradient necessary 

 to excite the tissue. This will be evident if we compare the maximum 

 summation intervals for different tissues with the excitation time of the 

 same tissues. 



Stimulating current 5% above minimal stimulus 



Summation interval !' Excitation time " 



sec. sec. 



Frog's nerve . . . -0005 . . -003 



sartorius .. . -0015 . . -017 



heart . . . '0080 . . 2-000 



REFRACTORY PERIOD. The phenomenon of a refractory period 

 has long been known in connection with the heart muscle and has 

 often been regarded as characteristic of this muscle. If, in the 

 isolated ventricle, a beat be evoked by a single minimal stimulus, 

 subsequent repetition of the stimulus during the course of the 

 contraction is ineffective, and becomes effective only when the 

 contraction has passed away. The heart is said to be refractory to 

 stimuli during this period. The duration of the refractory period is 

 a question of the strength of the stimulus used. With strong stimuli 

 the heart may be made to contract when the relaxation has only pro- 

 gressed half way, and with very strong stimuli one contraction may 

 be made to follow the last at such a short interval that hardly any 

 trace of relaxation is observable between the beats. The phenomenon 

 seems to be common to all excitable tissues. Thus if two stimuli are 

 applied to a nerve within a sufficiently brief interval, the second 

 stimulus is ineffective, so far as can be determined by the response of an 

 attached muscle or by means of a capillary electrometer. The period 

 is longer the lower the temperature and varies from -0006 sec. at 

 40 C. to -002 sec. at 12 C. This critical interval is lengthened if the 

 irritability of the nerve is depressed by narcotics. We may ascribe it to 

 the second stimulus being applied before the excitatory change due to 



