Fig. 91.
The same principles will explain the formation of new paths successively concatenated to no matter how great an extent, but it would obviously be folly to pretend to illustrate by more intricate examples. I will therefore only bring back the case of the child and flame (Vol. I. p. 25), to show how easily it admits of explanation as a 'purely cortical transaction' (ibid. p. 80). The sight of the flame stimulates the cortical centre S1 which discharges by an instinctive reflex path into the centre M1 for the grasping-movement. This movement produces the feeling of burn, as its effects come back to the centre S2; and this centre by a second connate path discharges into M2, the centre for withdrawing the hand. The movement of withdrawal stimulates the centre S3, and this, as far as we are concerned, is the last thing that happens. Now the next time the child sees the candle, the cortex is in possession of the secondary paths which the first experience left behind. S2, having been stimulated immediately after S1, drained the latter, and now S1 discharges into S2 before the discharge of M1 has had time to occur; in other words, the sight of the flame suggests the idea of the burn before it produces its own natural reflex effects. The result is an inhibition of M1, or an overtaking of it before it is completed, by M2.—The characteristic physiological feature in all these acquired systems of paths lies in the fact that the new-formed sensory irradiations keep draining things forward, and so breaking up the 'motor circles' which would otherwise accrue. But, even apart from catalepsy, we see the 'motor circle' every now and then come back. An infant learning to execute a simple movement at will, without regard to other movements beyond it, keeps repeating it till tired. How reiteratively they babble each new-learned word! And we adults often catch ourselves reiterating some meaningless word over and over again, if by chance we once begin to utter it 'absent-mindedly,' that is, without thinking of any ulterior train of words to which it may belong.
Fig. 92.
One more observation before closing these already too protracted physiological speculations. Already (Vol. I. p. 71) I have tried to shadow forth a reason why collateral innervation should establish itself after loss of brain-tissue, and why incoming stimuli should find their way out again, after an interval, by their former paths. I can now explain this a little better. Let S1 be the dog's hearing-centre when he receives the command 'Give your paw.' This used to discharge into the motor centre M1, of whose discharge S2 represents the kinæsthetic effect; but now M1 has been destroyed by an operation, so that S1 discharges as it can, into other movements of the body, whimpering, raising the wrong paw, etc. The kinæsthetic centre S2 meanwhile has been awakened by the order S1, and the poor animal's mind tingles with expectation and desire of certain incoming sensations which are entirely at variance with those which the really executed movements give. None of the latter sensations arouse a 'motor circle,' for they are displeasing and inhibitory. But when, by random accident, S1 and S2 do discharge into a path leading through M2, by which the paw is again given, and S2 is excited at last from without as well as from within, there are no inhibitions and the 'motor circle' is formed: S1 discharges into M2 over and over again, and the path from the one spot to the other is so much deepened that at last it becomes organized as the regular channel of efflux when S1 is aroused. No other path has a chance of being organized in like degree.
Fig. 93.
[430] Parts of this chapter have appeared in an essay called "The Feeling of Effort," published in the Anniversary Memoirs of the Boston Society of Natural History, 1880; and parts in Scribner's Magazine for Feb. 1888.