In action grown habitual, what instigates each new muscular contraction to take place in its appointed order is not a thought or a perception, but the _sensation occasioned by the muscular contraction just finished_. A strictly voluntary act has to be guided by idea, perception, and volition, throughout its whole course. In habitual action, mere sensation is a sufficient guide, and the upper regions of brain and mind are set comparatively free. A diagram will make the matter clear: [Illustration: FIG. 51.] Let _A, B, C, D, E, F, G_ represent an habitual chain of muscular contractions, and let _a, b, c, d, e, f_ stand for the several sensations which these contractions excite in us when they are successively performed. Such sensations will usually be in the parts moved, but they may also be effects of the movement upon the eye or the ear. Through them, and through them alone, we are made aware whether or not the contraction has occurred. When the series, _A, B, C, D, E, F, G_, is being learned, each of these sensations becomes the object of a separate act of attention by the mind. We test each movement intellectually, to see if it have been rightly performed, before advancing to the next. We hesitate, compare, choose, revoke, reject, etc.; and the order by which the next movement is discharged is an express order from the ideational centres after this deliberation has been gone through. In habitual action, on the contrary, the only impulse which the intellectual centres need send down is that which carries the command to _start_. This is represented in the diagram by _V_; it may be a thought of the first movement or of the last result, or a mere perception of some of the habitual conditions of the chain, the presence, e.g., of the keyboard near the hand. In the present example, no sooner has this conscious thought or volition instigated movement _A_, than _A_, through the sensation _a_ of its own occurrence, awakens _B_ reflexly; _B_ then excites _C_ through _b_, and so on till the chain is ended, when the intellect generally takes cognizance of the final result. The intellectual perception at the end is indicated in the diagram by the sensible effect of the movement _G_ being represented at _G´_, in the ideational centres above the merely sensational line. The sensational impressions, _a, b, c, d, e, f_, are all supposed to have their seat below the ideational level.