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294 W. MCDOUGALL : raise the general level of their tonus to a corresponding extent. 1 If the external sensory stimulus, in this case the noise, be a very powerful one, a single incidence of it may suffice to bring about the state just described, and the feebler it is the more frequently will it have to be repeated before it can effect this degree of change of state in the central nervous system ; just as, in the case of an animal deprived of the brain but with spinal cord intact, a sensory stimulus of only moderate strength must be repeated several times before it can excite a reflex movement, and can excite such movement through a single incidence only if it be of very violent character. 2 A similar cumulative effect of apparently ineffective or sub- minimal stimuli has been shown by Urbemtstisch to occur in the fully waking state when auditory stimuli of very low intensity are used. These cases of summation of effects of successive and separately ineffective stimuli are instances, I take it, of that important effect specially studied by Exner, 3 and by him named facilitation (Bahnung). I have already in another place attempted to show that this may be regarded as essentially a process of accumulation in neurones of successive charges of neurin individually in- sufficient to bring the potential of the neurones up to the discharging point. 4 The description of this first step of the awakening process, which brings us to the reflexly produced movement, presents no difficulties. But we have now to face the first of those four problems formulated at the end of the first part of this essay, 5 namely, ' Why is it that at any moment the excitation set up in the lower levels by some one of numerous simul- taneous sensory impressions penetrates to and excites an organised system of paths in the higher levels of the brain ? Why is the excitation-process not confined to lower-level paths of which the normal or resting resistance is lowest ? ' In the instance in hand, Why does the excitation-process, set up in sensory neurones by the auditory stimulus, not 1 Many persons and animals habitually stretch themselves on waking from sleep, powerfully innervating all their skeletal muscles, and this, as most of us know from our own experience, promotes very effectively the waking-process. It would seem that we share with the animals this instinctive tendency to secure in this way a rapid raising of the general tonus of the nervous system. 2 Sherrington, Art., " Spinal Cord," Schaefer's Text-book of Physiology, p. 828. 3 Entwurfz.physiolog. Erklarung psychischer Thatsachen. Wien, 1894. 4 Brain, Dec., 1901. 5 MiND, No. 43, p. 349.