. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . oot; and itcan, accordingly,be more readilyaroused in this lat-ter way than bystimulating thehind limb. Nowif both of these points—for example, the left forefoot and the right hindfoot—arestimulated at the same time, but so feebly that neither stimulus,taken alone, would arouse the movement of the fore limb, this limbmay, nevertheless, be made to move through the combined efifect ofthe two stimuli. In general, distant stimuli may facilitate each other
. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . oot; and itcan, accordingly,be more readilyaroused in this lat-ter way than bystimulating thehind limb. Nowif both of these points—for example, the left forefoot and the right hindfoot—arestimulated at the same time, but so feebly that neither stimulus,taken alone, would arouse the movement of the fore limb, this limbmay, nevertheless, be made to move through the combined efifect ofthe two stimuli. In general, distant stimuli may facilitate each othersaction. The impulses started at two widely separated receptorsmay converge upon the same central cells, and so produce move-ment of the same muscles. Even an auditory stimulus may facili-tate a reflex in the hind limb. In man, the knee jerk is strengthenedby a sudden noise or other stimulus which has the effect of startlingthe subject of experiment. Mental states may exert a similar reinforcement on various re-flex functions of the nervous system; and artificial stimuli applied See Lombard, American Journal of Physiology, 1887, I, Fia. 63.—Eeinforcement of the Knee Jerk Giving Way toInhibition. (Bowditch and Warren.) Distances along thehorizontal line represent the time elapsing between theclenching of the fist and the tap on the tendon whichevoked the jerk. Distances above the normal line rep-resent the amount by which clenching the fist increasedthe movement of the foot, and distances below the linerepresent the amount of decrease. The increase givesway to decrease at about sec. 172 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM to the cortex of the brain may facilitate the reflex effect of stimula-tion to the skin. § 28. This facilitating effect is, however, customarily one ofbrief duration; and it probably is subject to the diphasic law whichwas noted above (compare p. 165 and see Fig. 63). That is to say,the reinforcement soon gives way to its opposite, inhibition,
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