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of both. Indeed, a concurrent impulse may be applied to each wheel; and in any given revolutions the eye may not be able to decide in which the balance of force or the entire force is to be found. The relation, however, between the mind and the cerebrum is not doubtless of that exactly reciprocal, equivalent character implied by the comparison; the two do not stand to each other on the same plane of causation, nor is it in their normal action a matter of indifference by which of the two the power is applied. Their real relation to each other is complex, can only be disclosed by a consideration of the facts, and then but partially.

Activity of brain always accompanies activity of mind. (1) This is shown by the destruction of brain-tissue incident to energetic thought. A sensible increase of the waste due to nervous tissue attends on such action. We instantly conclude, and probably correctly, that the correspondence indicated is complete, and that all mental effort has an exactly equivalent expression in brain-action, and so in decomposition. (2) The fatigue and nervous exhaustion that accompany thought contain the same conclusion. We attribute these to the expenditure of nervous energy, with its destruction of tissue. (3) The renovation incident to rest sustains the argument. Sleep is the best restorative of mental functions, and sleep seems to bring peculiar repose to the brain through repose of the voluntary powers, and so to make way for nutrition and refreshment.

The reverse of this proposition is not so evident, that all activity of the cerebrum is accompanied by activity of mind. (1) We should not know what mental state, for instance, to refer to the reconstructive processes that proceed in sleep or in restful hours; (2) nor what modifications of thought to ascribe to incipient disease of the brain that