432 W. L. MACKENZIE :

to move the patient moves as directed ; when her movement is resisted unknown to her, she imagines the intended movement accomplished ; she has a feeling of energy given out. Surely this fact is not less important than the ignorance of position ; and we imagine this fact tells in favour of a motor concomitant of volition. It may be that this feeling of energy given out is due to afferent processes ; but there is nothing in the record to suggest that, and Dr. Bastian nowhere proves it. To one or two other cases almost the same remark applies ; they illustrate and almost demonstrate the ignorance of position due to paralysis of afferent nerves, but they demonstrate nothing regarding the central concomitant of conscious movements. (b) The distinction of Sensory and Motor. In spite of Dr. Hugh- lings Jackson's protests long ago, recent discussion has been too much bound to this couple. The distinction, as Prof. Haycraft insists, is a purely provisional one. "If at any time certain cells in this intercommunicating network were looked upon as the special seat of sensation and others as the seat of motion, it is impossible so to view them now. As a result of the passage of an impulse through the nervous system, we may have muscular movement and we may have sensation, but in the nervous paths through which the impulse passes it is not possible to say that one part is more motor than another, or to localise sensation to a given spot. . . , The cells in the cortex are on a loop between the sensory and motor cells of the cord. . . . They are trophic, and perhaps they act like collections of combustible materials placed on a train of gunpowder." This agrees with what Dr. Jackson says that the physiological substratum of every mental process is a sensori-motor process. And it seems not unlikely that sensory and motor processes will ultimately be expressed in terms of a more fundamental distinction the anabolic and cata- bolic changes of nerve-protoplasm. Already the process of inhi- bition is regarded as probably a building-up or anabolic change. (c) Relation to Consciousness. In arguing against the concomi- tance of feeling and central nervous discharge, it is commonly assumed that afferent processes are always accompanied by consciousness. But to the physiologist consciousness is a shifting affair. Not all processes that reach the cortex are conscious ; and some are at one time conscious, at another not. Yet in the general statement of the concomitance of conscious- ness with afferent processes, this variation does not count. Similarly with motor processes. The problem is not how much at any given moment is in the field of consciousness, but what must we imagine the physiological substratum of a given state of consciousness to be. The question of concomitant consciousness is in fact a case of " cerebral time " : at what point in the sensori- motor process does consciousness arise. If, with Dr. Terrier, we confine consciousness to sensory processes, then, since sensory may excite motor processes, we must imagine consciousness suddenly ceasing on the nervous bridge the internuncial fibres