91. We speak loosely of an organ being developed by increased activity; but this is loose speech, and investigation shows that the organ is not developed _by_, but accompanies the increased activity, every increment of activity being necessarily preceded by a corresponding increment of structure. This is evident _à priori_: the force manifested is inherent in the structure manifesting it. Thus we ought not to say “the vascular system furnishes good instances of the increased growth that _follows_ increased function”; we ought to say, “that _permits_ increased function.” The muscle having a contractile power represented by 10, expends, we will suppose, 7 units of force in its normal activity, and these are replaced by its normal nutrition. If from an extra demand upon it 9 units are expended, the muscle becomes fatigued, if 10, exhausted, and it will no longer contract, the whole disposable sum of its contractility being dissipated. During all these stages the structure of the muscle--or to prevent all equivoque, let us say the substance of the muscle--has been changing, not indeed in any degree appreciable to the eye, but appreciable by the more decisive tests of chemical and physiological reactions. Yet inasmuch as in the ordinary course of things the waste is quickly repaired, the muscle in repose once more regains its original state, once more represents 10 units of contractility. Now let us consider what takes place when extra labor is thrown upon the muscle, when exercise causes growth. At the outset of a walking tour we may not be able to compass more than twenty miles a day, at its close we manage thirty. Is it the increased activity of the function which has caused this increase of structure? In one sense, yes; but let us understand it. Had the increase of activity been temporary, there would have been only a temporary increase of structure. But when the ordinary expenditure of 7 units rises to 9, on several successive days, this extra expenditure of tissue has had to be met by an extra nutrition--i. e. more plasmode has been formed and more protoplasm. It is a physiological law, easily explained, that, within due limits, extra waste brings about extra repair: as the channels are widened and multiplied, the _derived_ currents become stronger, and the increased flow of nutrition which was temporary becomes permanent, because this increase is no longer dependent on an extra stimulus, but on an enlarged channel.[42] When the channels have not become multiplied or enlarged, which must be the case whenever the extra stimulus is fluctuating and temporary, the extra expenditure is not followed by increased size of the muscle: the currents resume their old directions, no longer being diverted.