89. It has long been one of the unquestioned postulates of Physiology that no nerve-action can take place without the intervention of a centre; and as a corollary, that all movement has its impulse--reflex or volitional--from a centre.[115] The postulate rests on the assumption that nerves derive their “force” from their centre. This assumption we have seen to be erroneous. Yet, in consequence of its acceptance, experimenters have failed to notice the many examples of nerve-action independent of centres. Indeed, except Schiff, Goltz, and Engelmann, I can name no one who has ventured to suggest that movements may be excited through nerves without the co-operation of centres;[116] nor have even they explicitly formulated the conclusion to which their observations point. It is true that the majority of muscular movements are determined by a reflex from centres; and that any break in the triple process of the ingoing nerve, centre, and outgoing nerve, prevents such movements. It is true that the more conspicuous and harmoniously co-ordinated phenomena belong to this class. But it is also demonstrable that many nerve-actions _may_, and some _do_, take place by direct stimulation of the nerve, or direct stimulation of the muscle, without the intervention of a centre, without even the intervention of a ganglion. This must obviously be the case in animals which have no centres; and even in some which have well-developed nervous centres, there is every reason to believe that these centres often act rather in the way of co-ordinating than of directly stimulating actions. 90. I was first led to doubt the reigning doctrine by a surprising observation (frequently repeated) after I had removed the whole nervous centres from a garden snail (_Helix pomatia_). The muscular mass called “the foot” was thrown into slow but energetic contraction whenever the skin was pricked with the point of a scalpel, or touched with acid; nay, even when a glass rod dipped in the acid was brought close to, without absolutely touching, the skin, the foot curled up, and then slowly relaxed. The same effect was produced on the “mantle”--where there was of course no centre. But direct irritation of the muscles under the skin produced no such contraction; only through the skin could the stimulation take effect. In one case I observed this strange phenomenon five hours after removal of the centres. It was a great puzzle. At first I concluded that there must be minute ganglia in the skin, serving as reflex-centres. I searched for them in vain; and although a longer search on better methods might _possibly_ have detected ganglionic cells, I soon relinquished the search, because I had other grounds for believing that even the presence of abundant ganglia would not suffice, until some better proof were afforded that such ganglia were reflex-centres.