These are remedies which impart permanent strength to the body, or its parts. When an individual is loose and limp, and feels unfit for work, like a relaxed bowstring, tonics restore his energy and strength, and again fit him for work. As their action in this respect resembles the effect of tightening a bowstring, they have received the name of tonics, which is derived from rovos, tension. The feeling of debility may depend on many different causes. It may be due to weakness of the muscles, or weakness of the nervous system. Again, the nerves and muscles may suffer because the circulation is languid and feeble, or because the blood which supplies them is deficient in oxygen, or in nutritive matter. These deficiencies again may depend on deficient nutrition, due to want of appetite, so that too little food is consumed, or to an improper or insufficient diet, or to imperfect digestion, so that the food is not assimilated. But weakness may be also induced by the accumulation of waste-products in the body, which interfere with the functional activity of the muscular and nervous systems; and these products may accumulate, because they are formed in excess in the tissues themselves by overwork, or in the intestinal canal from imperfect digestion; or because they may be allowed to pass too readily from the intestinal canal into the blood by deficient action of the liver.

Or their excretion may be defective from the kidneys being insufficiently active, or the bowels constipated.

The mode of action of tonics is so manifold that they have been divided into blood tonics or haematinics, vascular tonics, gastric tonics, intestinal tonics, and nervine tonics.

Uses. - In order to ascertain what form of tonic is required, it is necessary to determine carefully what part of the organism is in fault. In very many cases the imperfect functional activity in the body generally, which exhibits itself in languor and weakness, is due to accumulation of waste-products, and not to deficient nutriment. In such cases the plan of loading the stomach with food, and giving iron, wine, and beef-tea, simply increases the mischief. If it is found, on examination of the urine, that the kidneys are not excreting a sufficient quantity of solid, and especially of urea, it is necessary to diminish the quantity of food, and especially of animal food, as all, or nearly all, the nitrogen taken into the body must be excreted by the kidneys.

In order that no unnecessary work be thrown on the kidneys, we must, as far as possible, prevent products of imperfect digestion from being absorbed from the intestinal canal, and therefore the state of the liver must be carefully attended to, and the bowels themselves carefully regulated.

In cases where the debility does not depend upon excessive waste-products in the blood and tissues, but upon defective oxidation due to deficiency of haemoglobin, the patient must be treated by haematinics such as iron, cod-liver oil, and phosphate of lime. When the digestion is imperfect, gastric or intestinal tonics must be used as the case requires.

Where enfeeblement of the stomach appears to be present, as shown by loss of appetite, and such signs of imperfect digestion as flatulence or weight and pain after eating, gastric tonics are used. Should the muscular coat of the stomach be feeble or inactive, as shown by tendency to dilatation and splashing of the contents on movement, strychnine is especially indicated, and galvanism or systematic kneading may be also employed. Where the stomach is too debilitated to respond sufficiently to this form of treatment, as after long-continued gastric catarrh, or in old age, its work must be partly done for it, and then such digestives as hydrochloric acid and pepsin are useful. When the muscular movements of the intestine are sluggish, as indicated by constipation and by a tendency to distension of the bowel with gas, nux vomica and belladonna may be given; and when its mucous membrane appears to be relaxed and flabby, and secreting too profusely, the mineral acids, astringents, and metallic salts may be of much service. When the pulse is soft and feeble, and there is a tendency to vascular dilatation, either general or local, as shown by local congestion and oedema of the dependent parts, or by drowsiness in the upright position and sleeplessness in the recumbent posture, vascular tonics are serviceable. Nerve tonics are used where the nervous functions are imperfectly performed, as shown by dulness, loss of memory, incapacity for work, languor, paralysis, or tendency to spasm, as in chorea. As the functions of the nervous system depend very greatly upon the quality of the blood with which it is supplied, and on the rapidity of the circulation, the other tonics frequently require to be given in addition to nervine tonics.