The effect of drugs varies very much according to the quantity employed. Sometimes this is due to the interaction of different parts of the body on one another, as already mentioned in regard to veratrine (p. 26). Sometimes it is due to the different effects upon individual cells or tissues. Thus we find, very generally, that any substance or form of energy, whether it be acid or alkali, heat or electricity, which in moderate quantity increases the activity of cells, destroys it when excessive.

But varying doses do not always produce opposite effects. We sometimes find that exceedingly small and exceedingly large doses have a similar effect, which differs from that produced by moderate doses. Thus very minute quantities of atropine render the pulse somewhat slow; larger quantities make it exceedingly rapid, and very large quantities again render it slow.

Moderate quantities of digitalis slow the pulse, larger quantities quicken it, and still larger quantities render it slow again. We find a similar effect produced by variation in temperature. Great cold disturbs the mental faculties, so that men exposed to it present symptoms which cannot be distinguished from those of intoxication. Ordinary temperatures do not disturb the functions of the brain, but high temperatures do, as we see in the delirium of fever, which in many cases immediately ceases when the temperature of the patient is reduced by cold baths.