We may define fatigue as exhaustion from too much exercise or from too much work. Bunge aptly states that "the sensation of fatigue is one of the safety valves of our nature," which is a remark we may also apply to horses. If an extract of fatigued muscle be injected into the muscles of a healthy and unfatigued animal, it will produce symptoms of fatigue, which fact tends to prove that the feeling of fatigue is due to the formation, during exercise or work, of a poison or poisons in the system. Bodily exertion carried to the extent of fatiguing an animal, interferes with the healthy performance of excretion; the blood becomes loaded with carbonic acid, on account of the lungs not being able to give it off into the air with sufficient quickness for the requirements of health; breathlessness ensues; and congestion of the lungs may supervene in extreme cases. Ordinary breathlessness is therefore the poisoning of the body with carbonic acid which the action of the muscles causes to be produced at a faster rate than the lungs can remove from the system. In less violent work, fatigue may take place without breathlessness occurring. By practice, the organs of breathing acquire the ability to regulate their action, so that breathlessness may be prevented as far as possible.

The nitrogen of the broken-up nitrogenous matter which is taken into the blood, is normally found almost entirely in the form of urea (CH4N20), which is very soluble in water (dissolving in its own weight of even cold water), and is consequently a harmless product which the kidneys can readily remove from the system, supposing that these organs are in good condition. From the investigations of Roussin (see Colin's Physiologie Compare) it appears probable that during fatiguing exertion, a portion of the urea in the blood becomes replaced by hippuric acid (C9H9N03), which is sparingly soluble in water. Uric acid (C5H4N403), which is another form of nitrogenous waste, is regarded by Haig as the poison in the blood which gives rise to the feeling of fatigue in mankind. Colin tells us that it is present in abnormally large quantities in the blood of animals which are suffering from inflammatory diseases. Uric acid when comparatively abundant in the blood of man, produces great depression, and also gives rise to rheumatic affections, chiefly in the joints, by becoming deposited in these parts, or by uniting with the sodium of soda salts that are in the blood, and forming "chalk-stones" composed of hydrosodium urate. This salt and uric acid dissolve only to a small extent in water, and their comparative insolubility increases according as the temperature of the blood becomes lowered. Halliburton considers that fatigue is caused by the presence of sarco-lactic acid (C3H603). Whatever may be the poison in question, the fact remains that the imperfect elaboration of nitrogenous waste is an injurious result of fatigue, and that a plentiful supply of drinking water is necessary for the removal of products thus formed.

The degree of thirst brought on by fatigue is a reliable measure of the need which the system of a tired horse has for water. We find, in training either a man or horse, that the thirst after hard exercise or hard work gradually diminishes as "condition" is attained, which fact is a proof that exercise purifies the system.

In mankind, salicylic acid and also salicylate of sodium act well in removing the depression caused by fatigue, and in reducing the temperature in fever. Haig, Latham and others attribute their good effects in such cases to their power of removing uric acid from the blood by converting it into a soluble form. Whitla (Pharmacy, Materia Medica and Therapeutics) has obtained equally good results in the horse from 1/2 oz. doses dissolved in water and given from three to six times during the twenty-four hours. Salicylate of sodium, which is the best form of the drug, would certainly be preferable to the time-honoured ball of physic that is often given to horses out of condition, before putting them into work. Haig recommends that during four or five days before the commencement of violent work in the case of a man, uric acid should be cleared out of the system by salicylate of sodium, so as to obviate the ill results of fatigue. The administration of this drug should be stopped for at least a day before the work begins. The comparatively high price of salicylate of sodium is an objection against its employment in a stable, except, of course, in the case of valuable horses.

The poisonous influence of fatigue injuriously affects all the functions of the body. Hence we find that when a horse is tired, his powers of digestion are more or less impaired. Consequently he should get less food than usual, and the food should be of an easily digestible nature.