210 



IRRITABILITY 



than that resulting when a single stimulus is applied. These facts 

 have been known for a long time in the case of the heart muscle. 

 A word is necessary, however, concerning the effect of stimuli 

 beneath the threshold in heterobolic systems. We must here 

 distinguish between the "ideal" threshold, beneath w^hich the 

 influence of a stimulus is nil, and the threshold of perceptible 

 effect, beneath which a stimulus apparently has no effect ; never- 

 theless a weak effect does occur, as is shown by succeeding re- 

 actions. This effect is manifested by a sub-threshold disinte- 

 gration and a corresponding slight reduction of irritability. 

 (Figure 51.) The presence of such a sub-threshold effect is 



Fig. 51. 



Effect of sub-threshold stimuli, o— Level of the ideal threshold, 

 s — Level of the threshold of perceptible effect. 



recognized by various facts as, for example, the summation of the 

 sub-threshold stimuli to production of a perceptible result. Thus 

 stimulation of a sensory spinal cord root with a single sub- 

 threshold induction shock will not produce any evidence of a 

 reflex excitation, whereas, when induction shocks of the same 

 strength and of sufficient frequency are applied, a strong reflex 

 contraction results. The fact that sub-threshold stimuli can bring 

 about sub-threshold effects is also important in consideration of 

 the result of interference. The relation between the intensity of 

 the second stimulus and the degree of irritability of the system, 

 the intensity of the stimulus being absolutely constant, depends, 



