Adequate stable ventilation is nowadays recognized as essential for the maintenance of good health in the stud. No horse can be thoroughly well or fit, or in condition to do hard work or to resist disease, that is condemned to inhale the impure air of a badly-ventilated stable. When the inspired air is charged with equine exhalations, oxidation of the blood is lessened, elimination of impurities from the body is retarded, the system becomes loaded with waste products, and the vital force is markedly lowered. The visible results are that horses so housed become languid, easily fatigued, and show a marked tendency to succumb when attacked by any serious disease.

If horses are to be kept in good health the air they breathe must be pure, hence the necessity for ventilation, or, in other words, the extraction of impure air and the introduction of fresh air. This exchange requires to be done without unduly lowering the temperature or creating draughts, and it should be constant and regular. With the view of best securing this, many plans have been tried, but their efficacy depends on many extraneous circumstances, such as, e.g., the season of the year, the position of the stable, its size, etc. The inlets and outlets require to be much greater in hot than in cold weather, and in confined, closely-inhabited town positions than in thinly-populated exposed country districts. Regulation of temperature and prevention of draughts are more easily secured in small than large stables; and as the spread of infectious and contageous diseases takes place more readily in stables where large numbers of animals are kept, the majority of horse-owners are beginning to recognize the advantages of small over large stables. The entrance of fresh air is usually arranged for by means of gratings, and by tubes in the walls, by the doors, and by specially-constructed windows. The exits generally consist of extraction - shafts, patent cowls, gratings, windows, and louvred arrangements. But whatever plan of inlet and outlet is employed the former should be fairly low down and so placed as to avoid projecting draughts on any of the horses, and the latter should be high up in the building. The old principle of low inlet and high outlet is correct, and, when followed, a more thorough exchange of air is secured than when both inlet and outlet are placed on nearly the same level, for in the latter case the lower stratum of air surrounding the horses remains practically unchanged.

Likewise, whatever plan of exit and entrance is used, there should always be provision for regulating the size of the ventilators according to requirements, and due care should be taken that their proper adjustment be systematically attended to. The necessity for this will be fully apparent when it is remembered how much smaller apertures suffice for half-filled stables during cold windy weather than for well-filled stables during hot sultry weather.