The term reaction, when applied to conditions of health, signifies the more or less rapid readjustment of a functional balance which has been recently disturbed, as for instance, by going out for a few moments into sharp, frosty air, or by taking a plunge into cold water. If the action of cold on the skin in either of these cases produces a healthy glow over, respectively, the surface of the face or that of the body, we have healthy reaction, which is often accompanied by increased functional activity. If, however, the application of cold is too violent or too prolonged for the reaction to take place, chilli with its well-known bad results, will probably ensue. The greater the previous functional activity, the quicker will be the reaction, and the less will be the danger of chill taking place. Hence, when a horse is hot from recent exercise, he will have less chance of catching a chill from, for example, drinking cold water, or being made to swim in a river, than after he has cooled down. Although I have never known or heard of any harm having been caused to a horse by drinking cold water when he was in a heated state from exercise, the fact remains that men have dropped dead from nervous shock in similar circumstances.

When a horse "breaks out" into sweat after he has become cool, he is far more liable to chill, than when he returns to his stable hot from work; because, in the former case, his skin is colder than in the latter. "Breaking out" is caused chiefly by influences which - like hot stables and too much clothing - stimulate the sweat glands more than they increase the temperature of the surface of the body. Removal of the cause, exercise and vigorous friction to the coat, are evidently the appropriate remedies.

Disease set up by chill is a result of a change in the quantity of the local blood supply being carried beyond a healthy limit, as often happens when the surface of the body is rendered more or less bloodless by the continued action of a cold current of air, with consequent congestion of one or more of the internal organs. A cold draught playing on a horse in a stable is far more likely to give him a chill, than exposure to an equally cold and equally strong current of air in a field, where the effect of the wind would be much more general, and where he would have an opportunity of equalizing the distribution of blood in his body by exercise.

When the blood vessels have been deprived of their normal supply of blood for a considerable time, the blood, if suddenly restored to them, appears to act more or less as an irritant to their walls; the only safe way of overcoming the difficulty being to gradually accustom them to its return. Thus, in frost-bite, if the blood, after having been driven away by cold, is allowed to suddenly return, violent inflammation, which may be followed by death of the part, will ensue. Here, the proper treatment will be the adoption of means, such as rubbing with snow, which will permit of only a very gradual return of a full supply of blood. The usual effect, however, of cold is, as we have seen, to produce anaemia (not inflammation), and the special danger to be feared is congestion of the deep-lying parts. Hence, when horses are over-heated from exertion, we should have them walked about, or should have vigorous and prolonged friction applied to their skin by two or more strappers to each animal, so that the healthy congestion of the muscles of the limbs and of the superficial muscles of breathing may not be relieved too quickly; and we should stimulate the sweat-glands by giving a liberal supply of water. Neglect of these precautions is a frequent cause of congestion of the lungs, especially among unfit hunters which have to return by rail to their stable after having been out with the hounds. In a typical case of this kind, when the animal is pulled up and is freed from his burden, the blood vessels of his limbs and of the surface of his body are filled with blood which has been brought in response to the stimulus of exercise. This healthy congestion soon becomes changed to anaemia by cold air, and especially by a cold wind acting on a horse which is standing still in a railway horse box, or perhaps in a more exposed situation, and the internal organs now become congested. Probably, when the horse gets into his stable, the amount of grooming he receives will not exceed that which is sufficient to put his coat "straight"; he gets very little water; and we need not be surprised if by bedding-down time he is "blowing," and is in need of the prompt attendance of a veterinary surgeon to treat him for congestion of the lungs. In these cases of congestion from chill, the more an internal organ has been weakened by severe exertion or other causes, the more liable will it be to suffer. Thus we find in India that chill often produces congestion of the liver; but rarely that of the lungs; the opposite being the case in England. I may explain that the climate of India, differing from that of this country, is as a rule, very favourable to the well-being of the organs of breathing, but is highly detrimental to that of the liver. In India, acute attacks of liver disease are frequent just after the rainy season, in localities where there is a considerable fall in the temperature of the air at night, as in places close to the hills. This tendency is increased by the practice pursued by many of the native grooms, who are generally but ill-provided with garments, of removing the horse's clothing to use as their own bedding. It is easy then to conceive how the horse gets derangement of the liver; for the cold, acting on the surface of the body, contracts the superficial blood-vessels, and thereby drives the blood to the internal organs.