§ 23. =The Nervous Correlate of Attention.=—It remains to say a word about the nature of the nerve-forces (§ 20) which underlie attention. Physiologists tell us that one nervous process may influence another in two opposite ways: by helping and by hindering, or, in technical terms, by =reinforcement= and =inhibition=. Let us take an elementary example of what they mean. Suppose that a frog has been reduced, by the removal of its cerebral hemispheres, to a mere nerve-and-muscle machine; it lives, but it cannot sense or feel, and it does not move ‘of its own accord.’ If, now, a weak pressure is applied to the frog’s hind foot, there is no visible response; the limb remains passive. But if at the same moment a light is flashed into the eye, the leg-muscles may be thrown into strong contraction. Here we must suppose that the two nervous processes, from skin and eye, have in some way helped each other; there is nervous reinforcement. If, again, a pressure is applied to a certain part of the frog’s body, the animal croaks. If a strong pressure is applied to another part of the body, it replies by a contraction of the muscles. If, however, the two pressures are applied together, the frog does not both croak and move; it does neither; there is no response to the stimuli. Here, therefore, we must suppose that the two nervous processes interfere with each other; there is nervous inhibition. It seems plain that these two influences are at work among the nervous processes correlated with attention. _The vivid sensations at the focus are sensations whose corresponding nervous processes have been reinforced, and the dim sensations of the background are sensations whose corresponding nervous processes have been inhibited._ No doubt, the distribution of these forces, in a given instance, is really a matter of _degree_; the reinforced nervous process receives more reinforcement than inhibition, and conversely. No doubt, also, the _removal_ of an existing inhibition may produce the same effect as the addition of a reinforcement, and conversely. We are still too much in the dark as regards the intimate character of the nerve-forces, we know too little of their actual course as nervous function in nervous structure, to be able properly to distinguish cases. There is evidence that _inhibition_ may be extraordinarily effective: thus the late Dr. W. B. Carpenter relates that he “has frequently begun a lecture, whilst suffering neuralgic pain so severe as to make him apprehend that he would find it impossible to proceed; yet no sooner has he, by a determined effort, fairly launched himself into the stream of thought, than he has found himself continuously borne along without the least distraction, until the end has come, and the attention has been released; when the pain has recurred with a force that has overmastered all resistance, making him wonder how he could ever have ceased to feel it.” _Reinforcement_ also may be carried to a high degree: how else