306 PHYSIOLOGY 



limited to about '001 sec. From a number of experiments of this 

 description Lucas gives the following as the characteristic, or what 

 he terms the " excitation times," of muscle and of nerve : 



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 steeper, 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 pro- 

 duced 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. 



SUMMATIO ; OF STIMULI. Closely associated with the excita- 

 tion time of the tissues are the phenomena of ' summation of 

 stimuli ' and ' refractory period.' If two subminimal stimuli are 

 sent in within a sufficiently short interval of time, their effect is 

 smnmated, so that two stimuli, each of which would be ineffective, 



