272 PHYSIOLOGY 



Muscle . .... '017 sec. 



Nerve fibre ... . '003 



Nerve-ending (or intermediary substance) "00005 



Similar differences are obtained when we attempt to determine by means of the 

 rheonome the minimal gradient of current necessary to excite a nerve. In the case 

 of the toad's nerve the minimal gradient must be ten times as steep as in the case of 

 the toad's muscle, and is such that in one second the current must reach a strength 

 forty-five times the minimal strength which is necessary to excite when the current 

 is made instantaneously. In the frog's nerve the minimal gradient is still sleeper, 

 so that in one second the current must reach sixty times the strength of the minimal 

 exciting current. We may interpret these results as signifying that the excitatory 

 state produced in an irritable tissue involves the production of some change which 

 passes away spontaneously. The rate of production of the change, and still more the 

 rate of its spontaneous disappearance, differ from tissue to tissue. If the gradient 

 of a current which is made through the tissue is too slight, the spontaneous disappearance 

 of the excitatory change goes on as rapidly as the production in consequence of the 

 rise of current. No excitation therefore takes place. The ' excitation time ' of the 

 tissue will thus be proportional to the duration of the excitatory change produced in 

 the tissue as a result of the stimulus. We may compare the excitation time of three 

 tissues with the duration of the electrical change produced in the same tissues by a 

 single stimulus. 



The excitation times were : 



Frog's nerve fibre ... . '003 sec. 



Muscle fibre of sartorius .... '017 

 Ventricular muscle of frog .... 2*000 



In the case of muscle, according to Burdon Sanderson, the electrical change reaches 

 its culminating point in '0025 sec., and may take perhaps eight times this interval 

 before it dies away. In the cardiac muscle of the tortoise Sanderson found the electrical 

 change to last between two and three seconds. 



SUMMATION OF STIMULI. Closely associated with the excitation 

 time of the tissues are the phenomena of ' summation of stimuli ' and ' re- 

 fractory period.' If two subminimal stimuli are sent in within a sufficiently 

 short interval of time, their effect is summated, so that two stimuli, each 

 of which would be ineffective, may together produce an excitation. In 

 the case of striated muscles, in order that mechanical summation of con- 

 traction 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 summation 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 summa- 

 tion 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, 



