The tissues seem to have a certain power of adapting themselves to changes in their surroundings. Thus salt-water amoebae will die when placed at once in fresh water, but if the fresh water be added very gradually, they may by-and-by become accustomed to live in it. Fresh-water amoebae also have the power of becoming gradually accustomed to increasing quantities of salt gradually added to the water in which they live, and which would at once kill them if added suddenly. A similar power seems to be possessed by the tissues of the higher animals, in regard to some drugs at least. Thus the arsenic-eaters of Styria are able to consume - not only without injury, but with apparent benefit to themselves - a quantity of arsenic which would prove fatal to one unaccustomed to it. The same is the case with opium and morphine. With these latter drugs there seems to be hardly any limit to the quantity which can be taken after the habit has been once established, and after a certain dose has been exceeded.

It is possible, however, that in addition to a process of accommodation going on in the tissues, there is a slower absorption, and perhaps more rapid excretion, going on at the same time; for it is observed in the case of opium that sometimes the effect is not only diminished, but the time which elapses before it occurs is lengthened when persons have become accustomed to the drug.

In regard to the possibility of very slow absorption we must remember the power of the liver to arrest and excrete or to destroy poisons, especially as it is chiefly in the case of vegetable poisons that their power is lessened by habit, which has much less influence on the effect of inorganic substances. The tolerance of some inorganic drugs, and especially of tartar emetic in disease or after repeated doses, may be due to fever or the drug itself lessening the acidity of the stomach, and consequently the action of the drug, which acts most strongly in presence of an acid.