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 42 J. S. HALDANE. But it is not necessary to go out of the way to find in- stances which, it seems reasonable to suppose, are analogous. When, from valvular disease or other causes, increased work is required from the heart, it becomes hypertrophied. Similar hypertrophy in other muscular organs, and in volun- tary muscles, occurs under similar circumstances. When one kidney is destroyed the other becomes hypertrophied and does double duty. On the other hand tissues which from any cause have become useless tend to atrophy, as may be seen in the case of muscles which are not used, the collar- bone alter amputation of the arm on the same side, or the jaw-bone after the teeth have been lost. The restoration to their ordinary condition of organs whose structure has been profoundly altered by inflammation is almost as remarkable a phenomenon as any of those just mentioned. The more one considers such phenomena as these, even apart from other biological facts, the more does one feel the difficulty, if not impossibility, of putting on them any different inter- pretation from that which was put on the reproduction of the limb in the newt. But when the behaviour of the tissues generally in the lower animals, and that of the higher nervous centres in man, are also considered, it appears to me that there can scarcely remain any further doubt on the matter. Clear conceptions as regards this matter would seem to be of great importance in medicine. As has been seen already, the apparent action on an organism of one of its parts is just as much the action of the organism on the part ; and part and organism, as regards both what they are in themselves, and their apparent action and reaction on one another, only manifest in themselves the life of the organism regarded as a whole. In like manner a remedy cures diseases only in so far as its physical or physiological action is taken up into the life of the organism, so that that life becomes manifested in it. It is a common source of misconception that in the process of cure the remedy is regarded merely as a cause, producing a certain measurable effect in the diseased organism. To this source may be traced the fact that with the advance of pathological anatomy there grew up so much scepticism as regards therapeutics. For it is clearly impossible to replace artificially the ex- quisitely delicate tissues which are destroyed or injured by disease. But experience shows that remedies actually do most materially assist in the process of restoration of the function of a part of the body that has been injured or destroyed. For instance, when a valve of the heart has been injured by inflammation, and the circulation thus becomes inefficient, digitalis can be given, not merely in