246 CRITICAL NOTICES : simple, conceived of as in isolation from its connexions, is always, physiologically considered, a complex formation made up of various jaerve-processes spread over a large number of elementary parts". And it follows from his principle of adaptation that the whole -.sensory tract from sense-organ to brain-cortex, if not also the ^efferent tract by which the sensory excitation discharges itself, must be held by Wundt to be included in this " complex forma- tion " in every part of which the specific process, the immediate physical correlate of the sensation-element, repeats itself. Accord- ing to this view, then, sensation should accompany the process >even though it be confined to the initial part of the tract in the sense-organ, yet all the available evidence goes to show that the excitation must be propagated through certain definitely localised elements of the cerebral cortex in order that sensation shall be excited. Of all the special psycho-physical problems the most important, after that of the nature of the physical correlate of sensation, is the inquiry into the physiological conditions of retention and the reproduction of sense-impressions or percepts as images or ideas. For Wundt the sensation differs from the memory-image only in intensity, the latter is merely a sensation of low intensity, the pre- sentation, whether percept or idea, is a synthesis of such sensation- elements, and the percept differs from the idea of the same object only in that some of its elements are more intense. The substrate of the physical correlate of the presentation is therefore identical with that of its reproduction as idea and it consists of a conjunction of those " complex formations " in which the physical correlates of the sensation-elements run their courses. In the case of the percept this complex "functional disposition" is peripherally excited, in the case of the reproduction it is centrally excited. This conception of the excitement of a complex functional disposition underlying the rise to consciousness of both the presentation and its reproduc- tion is certainly very superior to that of " memory-cells" in which ideas may be deposited for future use, a conception which Wundt rightly rejects as impossible and misleading. The gaining of an insight into the mode of formation of such functional dispositions, made up of structures in widely separated parts of the brain, is one of the primary tasks of physiological psychology, but Wundt gives us very little leading here. We are told only that it must be an effect of physiological practice, and must be due to that property of nervous substance in virtue of which every excitation of it must leave behind a disposition to the recurrence of a similar ex- citation. What I have elsewhere called the crucial problem of physiological psychology, namely, the problem of the first forma- tion of new association-paths in the nervous system, is thus ignored, and the conception of the functional disposition is left so vague as to be of little value save as a corrective of cruder conceptions. Of other special psycho-physical conceptions the most important is that of the " apperception-centre " in the frontal lobe. This has
