The circumstances which are calculated to modify the ordinary and characteristic action of medicines should always be taken into account by the physician. These may be divided into such as relate especially to the medicines, and such as relate to the system. The former will be more conveniently treated of when the medicines themselves are considered. A few general remarks, in relation to the latter, will be appropriate in the present place. To enter into minute particulars would be quite impossible; as there is scarcely a change, whether consequent upon the healthy progress of the body from birth to old age, or upon the operation of morbid causes, which does not in some measure influence the effects produced by medicines. Many of these influences will be referred to in connection with the several medicines or remedies described; but much, in practice, must always be left to the observation. experience, and judgment of the physician.

The modifying influences may he such as are essentially connected with our bodily constitution in health, or such as are more or less accidental. The first may be ranked under the heads of age, sex, temperament, and idiosyncrasy; the second under disease, climate, habit, modes of life, and mental action.

1. Age. It is a general, though not universal law of nature, that susceptibility to the influence of medicine is inversely proportionate to the size of the animal. This probably results chiefly from the greater amount of a medicine required to give a certain degree of impregnation to the blood of the larger animal than the smaller. The medicine acts on different bodies, not in proportion to its absolute quantity in the blood, but to the quantity of it which is brought to bear upon each point acted on, in other words, to the strength of its solution in the blood. Under the rule here referred to, the child should be more susceptible to the influence of medicines than the adult, and should consequently be affected by smaller quantities. But there is another reason, also, for the greater susceptibility of early life. In the growing state, greater impressibility and mobility are essential, in order that there may be a more rapid assimilation of external material, and a due arrangement of the organism. The higher susceptibility to impression must extend to medicines, as well as to all other impressing agents. Still another cause of difference, in this respect, between the young and the old, is the absence or less degree, in the former, of the influence of habit in diminishing susceptibility. I do not here allude to the habit of using medicines; for the cause operates though no medicine may ever have been taken. The general impressibility of the system diminishes by time under the necessary influence of external agents; and this law holds good even in relation to particular agents to which the system may never have been exposed, though it would be less operative in reference to these than to others.

It is impossible to give any precise rule for proportioning the dose to the age; because different individuals exhibit a great difference in this respect; and there is a remarkable diversity in reference to medicines; some, as opium, producing in children more than the mean proportionate effect; others, as castor oil and calomel, much less. It may be said, in general terms, that the dose for an individual under maturity should he proportioned to the years of his age. This holds good in relation to all ages between 12 and 24, at the latter of which periods of life, the full dose may be given. From the age of twelve downwards to two years, the rule of Dr. Young is perhaps as good as any that can be given; namely, that the dose proper for an adult should be diminished, for a child, in the proportion of the age increased by twelve to the age. Thus, the age being three years, and the dose for an adult 20 grains, the diminution must be in the proportion of 3+12 = 15 to 3; or, the quantities being reduced to their lowest terms, of 5 to 1; that is, the dose for the child must be one-fifth of that for the adult, or in the present instance 4 grains. At one year the dose may be one-ninth; at nine months, one-tenth; at six months, one-twelfth; at three months, one-fifteenth; atone month, one-twentieth; but it must be admitted that these numbers are nothing more than safe approximations. From full maturity to the commencement of declining life, that is, from about twenty-four to forty-eight, the dose may remain unchanged; but, after the latter period, it should be somewhat diminished with the increasing age, not because the system becomes more susceptible; for, as a general rule, it is less so; but because it is less able to sustain, without injury, a given impression from medicines than in the full vigour of life.

2. Sex. It is necessary to say but little under this head. There are certain conditions in the female which require attention in the prescription of medicines, which, however, cannot be said to exert any materially modifying influence over their effects, and do not, therefore, require particular attention in this place. Such is pregnancy, which demands especial caution in the use of all medicines having a direct influence on the womb, and which, in its advanced stages, contraindicates the use of any medicine whatever of a powerfully perturbating character. Such, too, is the menstrual state, in which care is required, in the employment of remedies, to guard against any interference with the uterine function. Another important point, of a similar bearing, is the caution requisite, in the eases of nursing women, not to use medicines which might injure the suckling; and a similar caution may be very properly extended to pregnancy, in which, while prescribing for the female, we should always bear in mind that there is another being to be affected by the remedy employed.

So far as concerns the modifying influence of sex upon the effects of medicines, the only circumstance of importance is, that women, being smaller, more delicately organised, and in general more susceptible than men, require a smaller amount to produce a given effect. The dose for females should, therefore, be somewhat reduced. From one-sixth to one-quarter may be deducted for them from the dose proper for the male at the same age.