Drugs which conduce to sleep are called by the above names, and their action upon man is fairly uniform except where some special idiosyncrasy exists; but with regard to horses we are not so well informed. They are light sleepers as a rule, and in health seldom repose for longer than four or five hours. The nervous temperament of the animal does not permit him to abandon himself to that complete oblivion which is a more or less acquired habit by civilized man enjoying perfect security of life. Drugs which have a soothing influence upon the nervous system, and invite sleep rather than compel it, are called by the above names, but those which overpower the brain and master the inclinations are known as Narcotics. The same agent may act as a gentle soporific in small doses and as a powerful narcotic in large ones. Drugs which relieve pain conduce to sleep, the anodyne effect of opium having been understood perhaps longer than that of any other drug.
Chloral Hydrate, although discovered by Liebig in 1832, did not come into use in this country until quite thirty years later, and its value in veterinary medicine was not ascertained until some years after the medical profession had proved its efficiency as a therapeutic in human practice.
It is extremely distasteful to horses owing to the hot burning sensation it imparts to the mouth, and in a lesser degree to the skin also, while it acts as a powerful irritant to abraded surfaces.