PHYSIOLOGICAL FACTOES OF THE ATTENTION-PROCESS. 353 normal or resting resistance is lowered in proportion to the intensity of its excitement, then it follows that the excitement of any one such system must tend to inhibit that of every other, and that the system, which at any moment is rendered by a constellation of favouring conditions the path of lowest resistance from afferent to efferent side of the brain, will divert to itself and transmit to its efferent neurones the energy which is liberated in the afferent neurones of any other system simultaneously excited and so will inhibit that system. Now let us turn to the third line of reasoning which, as it seems to me, affords the strongest possible confirmation of the truth of this view of the inhibitory or negative aspect of the attention-process. When any two objects occupy the focus of consciousness in immediate succession, or in other words when the attention is turned directly from one object to another, they tend to become mentally associated so that the recurrence of the one tends to be followed by the recur- rence of the other. It is generally agreed that the physio- logical process corresponding to this establishment of a mental association is the formation of a path of permanently lowered resistance between the two groups of neural elements whose excitement corresponds to the appearance of the two )bjects at the focus of consciousness. Further, it is gener- ally agreed that the canalisation of such an association-path, the permanent lowering of its resistance, is the result of the transmission of the nervous impulse through it. That is to say, the excitement of one neural system, B, following immedi- ately upon the excitement of another system, A, determines the transmission of a nervous impulse through some chain of nervous elements connecting the two systems. The problem is to understand how this is effected how does the excite- ment of system B, immediately following the excitement of system A, cause an impulse to pass from A to B along some intervening chain of neural elements? This question has hardly been defined by most of the writers on physiological psychology, much less answered. The only satisfactory answer that has been suggested is that of Prof. James, namely B drains A. 1 I am here only attempting to show that this is the only possible answer, to define more clearly the nature of the process of drainage, and to show that it is but a special, though the most important, case of a process constantly going on in all the different levels of the nervous system, namely the process of inhibition by drainage. Since it must be admitted that the excitement of any 1 Principles of Psychology, chap, xxvi., final section.
