PHYSIOLOGICAL FACTOES OF THE ATTENTION-PROCESS. 359 of a muscular contraction, the essential effect is the issue of a fuller stream of energy from the system of cortical elements whose excitement initiates the movement and gives rise to the idea of the movement. And it seems probable that this fuller stream of energy is the result, not so much of a more active metabolism of the neurones of that neural system, but rather of a greater concentration of the free energy of all parts of the brain into those channels, into that system. For the degree of effect of this sort that can be achieved by voluntary effort depends upon the amount of that free energy present in the brain. Thus when I squeeze my dynamo- meter with maximal effort just after waking from a long sleep it registers only about 70 or 80 kilos., but if I repeat the effort a few minutes later after rising, going into the light and plunging into cold water, it registers about 120 kilos., a difference which implies a very great increase in the amount of energy discharged from the motor cortex. I suggest then that, while all attention involves concentration or convergence of free nervous energy from all or many parts of the brain into some one neural system, voluntary effort results in a further degree of this concentration of energy. I have tried to show how the concentration of energy of non-voluntary attention may be explained by purely physical principles, but I am not sure that my explanation completely accounts for it or that it can be completely accounted for on physical principles, and this is I think still more doubtful in the case of the higher degree of concentration that results from voluntary effort. It is here, if anywhere, that the inter- actionist must seek in the insufficiency of physical causes evidence of psychical efficiency; here possibly we have a residual effect which, as I suggested on an earlier page, may be evidence of psychical guidance of physical process. If such power of guidance of energy by psychical effort in how- ever small degree be granted to the interactionist, he has all that is needed for his purposes. The world is then not purely mechanical, and biological evolution may be regarded as increasingly teleological, as swayed in an ever-increasing degree by final causes, and we may hopefully look forward to the time when man will control by voluntary effort the further evolution of his species.
