the _“ideal” threshold_, beneath which the influence of a stimulus is nil, and the _threshold of perceptible effect_, beneath which a stimulus apparently has no effect; nevertheless a weak effect does occur, as is shown by succeeding reactions. This effect is manifested by a sub-threshold disintegration and a corresponding slight reduction of irritability. (Figure 51.) The presence of such a sub-threshold effect is 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, secondly, upon the momentary amount of irritability which exists just at the time when the second stimulus produces its effects. It is, therefore, clear that the response produced by interference must also alter with the momentary degree of irritability in a manner analogous with variations of the intensity of the second stimulus. One must, therefore, know the factors which control the momentary degree of excitation. [Illustration: Fig. 51. Effect of sub-threshold stimuli. _o_--Level of the ideal threshold. _s_--Level of the threshold of perceptible effect.] [Illustration: Fig. 52.]