PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 351 levels ; (3) the fact that its acceptance solves the problem of the establishment of associations in virtue of temporal conti- guity or succession of presentations, while its rejection leaves us without any explanation of this process, an understand- ing of which is of the first importance for physiological psychology. We may briefly consider each of these in turn. Prof. Sher- rington l has shown that any two systems of sensori-motor reflex arcs of the spinal level which control two antagonistic muscle-groups are in a relation of reciprocal inhibition, such that the stronger excitement of one inhibits completely the weaker excitement of the other, and that if the stimula- tions are continued the system which is at first inhibited may become active, inhibiting in turn the other more strongly stimulated system; and he has shown that if the sense- organs forming the sensory ends of the two systems are continuously and equally stimulated, the two systems may discharge themselves by their motor neurones, not simul- taneously, but in regular alternation, either one in turn inhibiting the other. The simultaneous stimulation of the two retinae by rays of different wave length (or intensity) produces, as we have seen above, exactly parallel results. The alternation of the two colour-sensations (or of the two achromatic sensations of unequal brightness) in consciousness shows that the more strongly stimulated sensory tract at first inhibits completely the other, and that, if the stimulation is continued, the latter becomes active and inhibits the former, while if both are stimulated continuously by rays of different wave length but equal intensity there results a regular alternation of activity of the two tracts. In an exactly similar way, on regarding such an ambiguous figure as fig. 7 (p. 482, vol. xii.) the regular alternation of the two modes of perception of the figure implies a regular alternation of the activity of two- systems of neural arcs in the upper levels of the brain, and if the activity of one is favoured in any way equivalent to more intense stimulation it continues for some time to be active but then comes to rest and gives place to the activity of the rival tract. In all these three cases, then, there iff evidence of alternating activity of two rival neural systems in relation of reciprocal inhibition, and the conditions which affect the alternations of activity and the predominance of one or other system are so similar that we can hardly doubt 1 Journal of Physiology, vol. xxix., p. 64, and a series of papers on the- " Reciprocal Innervation of Antagonistic Muscles," in Proc. Roy. Soc.
