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PHYSIOLOGICAL FACTOES OF THE ATTENTION-PEOCESS. 357 passing through such an arteriole (changes brought about by the products of metabolism of the neurones or by hitherto undiscovered vasomotor nerves) might play some part in the fixation or remission of attention. But the actual conditions are the extreme opposite of these, a single neurone is often inches or feet in length, and the neurones constituting a func- tional group or system are, probably in every case, intimately mingled and interwoven with thousands of others belonging to other systems, and the vascular supply of a single percep- tual system must be drawn from hundreds or thousands of arterioles and capillaries widely separated in space, coming from many different arteries and supplying also parts of very many other neural systems. VOLUNTARY EFFORT. Without entering into the pychology of volition we may briefly consider the nature of its effects on the process of attention. " The volition to attend," says Stout, "is strictly analogous to the volition to move the arm, or perform any other bodily action. It follows from this that our voluntary command of attention must depend on our voluntary com- mand of the motor processes of fixation." l And this volun- tary control of the motor processes, together with a possible but highly improbable control of undiscovered vasomotor mechanisms in the brain, he regards as the only means of voluntary control of attention. I would accept unreservedly the statement of the first sentence quoted above. The second sentence states concisely a widely current doctrine which, I submit, has no logical foundation. It would seem that this doctrine owes its prevalence to that way of thinking which I have criticised above (p. 333) ; direct volitional control of motor innervation is accepted as an indisputable and familiar fact, the inexplicableness of which is obscured by its familiarity, while volitional control of any other kind of neural process is denied because of its inexplicableness. That Dr. Stout should have found himself compelled to invoke a purely mythical voluntary control of vascular changes in the brain should lead us to suspect that he has left out of account some important factor. I think we may safely go farther than the first of Dr. Stout's sentences quoted above, and may say that, not only is the volitional innervation of muscles strictly analogous to volitional atten- tion, but that the former is merely a special case of the latter, that all volition works by way of concentration or intensi- 1 Analytic Psychology, vol. i., p. 243. 24