WILHELM WUNDT, Grwndzuge d. Physiologischen Psychologic. 243 -duction-hypothesis, gives us new and fruitful conceptions of the mechanism of the nervous system. According to this view the neurone is not merely a structural and trophic element, but is also the functional unit, and the study of the nervous processes becomes, not merely the study of the conduction of impulses through a vast network modified only by the presence of nucleated cell-bodies on the network, but rather the study of conduction in the neurone, of the reciprocal relations of the neurones and of the action of neurone on neurone across the synapse. This scheme of the nervous mech- anism, strongly based as it is by inference from many experimental -observations, is of the greatest value to the physiological psy- chologist, for it is much better suited to his needs than the older tconception of the nucleated network. Yet Wundt substantially retains this older view and accepts the neurone, not as a physio- logical theory, but as a histological hypothesis merely (i., 42). Hence he is led to form a number of hypotheses as to the struc- ture and function of the nucleated parts or cell-bodies which are not supported by any direct evidence and which afford less satis- factory explanations of the observed facts of central conduction than does the neurone-theory. An example may be given. Wundt .attributes the variable degrees of resistance presented by conduc- tion-paths to the substance of the nerve- fibres in general and to the nucleated parts in particular. But experiment has failed to prove any such resistance to the passage of an impulse in nerve-fibres or cell-bodies, and if, as Wundt teaches, the conduction of the im- pulse through the nerve-cell is essentially a process of explosive decomposition, it is difficult to see how it can offer resistance to the passage of the impulse any more than a train of gunpowder can offer resistance to the passage of the flash. To meet this difficulty Wundt invokes the aid of an hypothesis concerning the relation of anabolic and katabolic processes, which, though superior in some respects to Hering's well-known though little understood doctrine of metabolism, involves the same fundamental difficulties. The formation of habits and other effects of use and practice are regarded by Wundt as due to diminution of this resistance of the cell- substance, and are assimilated to the raised excitability ex- hibited by a nerve-fibre during a few seconds after stimulation. He thus identifies two effects which the neurone-theory would keep apart, namely on the one hand facilitation or Bahnuny, that tem- porary and fleeting increase of the excitability of neurones which Exner first demonstrated in the central nervous system and to which he first applied the term Bahnuny, and on the other hand the permanent lowering of the resistance of conduction-paths which the neurone-theory would attribute to some structural change in the synapses that connect the constituent neurones of the path, a change which renders the connexions more intimate and which is usefully distinguished from facilitation by the term canalisation. Here the neurone -theory has the further advantage over the network-theory that, the junctions between neurones being much more numerous
