352 w. MCDOUGALL : that the process of inhibition is of the same nature in all three cases. The difficulties of other current hypotheses of inhibition are very great in the case of inhibitory processes in the spinal and sensory levels, but when we seek to apply them to the explanation of inhibition in the higher levels, a further very great difficulty arises from the fact that every higher-level system seems to be capable of inhibiting every other one. If, with the majority of physiologists we assume that inhibi- tion is always effected by the transmission from the inhibiting to the inhibited group of neurones of a peculiar nervous im- pulse different in kind from, and opposite in effects to, the ordinary exciting impulse, then we have to assume that whenever such a higher-level system is excited it sends such inhibitory impulses either to every other such system, i.e. to every organised element of the brain cortex, or to every other system that is in any way excited or about to be ex- cited, cutting short or preventing its excitement. The former alternative implies an incredibly wasteful expenditure of energy, the latter leaves us with the insuperable difficulty of explaining how the inhibiting system finds out which other systems need to be inhibited in order that it may send its inhibitory impulses to them. It would seem that it was the consideration of the latter difficulty that led Wundt to the conception of an apperceptive centre or organ of inhibition in the prefrontal lobes of the brain ; this conception has found so little favour in either its psychological or its physiological aspect that it is not necessary to insist upon its unsatisfac- tory character in this place. The same dilemma meets us if, instead of assuming that the special inhibitory impulses cut short the excitement of all other systems, we suppose with Miinsterberg l that they in some way block the efferent outlets of all other systems, or of all that are in any way simultaneously excited. On the other hand, if, in accordance with the evidence adduced in an earlier article of this series,- we regard the neural systems of the upper brain-levels as so many organised channels by which a common store of free energy, contained in the mass of afferent neurones and maintained in them at a varying pressure or potential by the streams of energy flowing in from the sense-organs, is discharged into efferent channels ; and if also we assume, as we have seen good reason to do, that during the excitement of any such channel its 1 Gh-undzuge der Psychologic, Kap. 1 5, " Die Aktionstheorie ". a MiND, N.S., No. 47, vol. xii., p. 300.
