350 w. MCDOUGALL : paper l I have put forward an hypothesis as to the nature of the process, the hypothesis of inhibition by drainage, which may be briefly restated as follows : The excitement of any sensori-motor arc diminishes the resistance that it offers to- the onward passage towards its efferent neurones of the current of free nervous energy or neurin, and its resistance is the more diminished the greater the intensity of its excite- ment. If then two or more such arcs are connected together in their central parts, that one which is the most intensely excited will become for the time being the path of lowest resistance for the escape of neurin to the motor neurones, and will therefore tend to drain to itself and to discharge by way of its motor neurones the energy liberated in all the others. I have shown (1) that this hypothesis affords explanations of the facts of inhibition in the spinal cord at least as satisfac- tory as those of any other hypothesis and that there are great difficulties in the way of all the rival hypotheses ; 2 (2) that many of the peculiarities of visual sensation and perception,, including all the phenomena of light- and colour-contrast, 3 are due to inhibitory processes, occurring probably in the arcs of the sensory level, and that some of these, notably the phe- nomena of smoothly graded light contrast 4 and smoothly graded colour 5 contrast, the predominance of contours in bin- ocular rivalry and the paradoxical phenomenon of Fechner. & seem to be explicable in terms of the drainage hypothesis only. Here I wish to extend this hypothesis to the explanation of the relations of reciprocal inhibition which obtain, as we are compelled to infer from the peculiarities of the attention- process, between the systems of neural arcs of the higher brain-levels. This view is based upon evidence of three principal kinds : (1) The very close similarity between the reciprocal inhibitions that occur in the spinal and in the sensory levels on the one hand and the simplest instances of reciprocal inhibition in the higher levels on the other hand ; (2) the fact that the hypothesis of inhibition by drainage seems to be the only one that is in any degree adequate to the explanation of the inhibitory processes of the higher 1 " On the Nature of Inhibitory Processes in the Central Nervous Sys- tem," Brain, Summer Number, 1903. 2 Brain, 1903. 3 " New Observations in Support of Young's Theory of Vision," pt. i. r MIND, N.S., No. 37, vol. x. 4 Journal of Physiology, " Proc. of Physiol. Soc.," March, 1903. 5 Ibid., July, 1903. 6 Note in the British Journal of Psychology, vol. i., pt. i.
