The mucous membrane lining the nasal chambers, and, in fact, the entire respiratory tract, is much more delicate and more frequently diseased that the mucous membrane of any other part of the body. The sinuses of the head are compartments which communicate with the nasal chambers and are lined with a continuation of the same membrane that lines the nasal chambers; their presence increases the volume and modifies the form of the head without increasing its weight. The horse, in a normal condition, breathes exclusively through the nostrils. The organs of respiration are quite liable to become diseased, and, as many of the causes which lead to these attacks can be avoided, it is both important and profitable to know and study the causes. CAUSES OF DISEASES OF RESPIRATORY ORGANS. The causes of many of the diseases of these organs may be given under a common head, because even a simple cold, if neglected or badly treated, may run into the most complicated lung disease and terminate fatally. In the spring and fall, when the animals are changing their coats, there is a marked predisposition to contract disease, and consequently at those periods care should be taken to prevent other exciting causes. Badly ventilated stables are a frequent source of disease. It is a mistake to think that country stables necessarily have purer air than city stables. Stables on some farms are so faultily constructed that it is almost impossible for the foul air to gain an exit. All stables should have a sufficient supply of pure air, and be so arranged that strong drafts can not blow directly on the animals. In ventilating a stable, it is best to arrange to remove air from near the floor and admit it through numerous small openings near the ceiling. The reason for this is that the coldest and most impure air in the stable is near the floor, while that which is warmest and purest, and therefore can least be spared, is near the top of the room. In summer, top exits and cross currents should be provided to remove excessive heat. Hot stables are almost always poorly ventilated, and the hot stable is a cause of disease on account of the extreme change of temperature that a horse is liable to when taken out, and extreme changes of temperature are to be avoided as certain causes of disease. A cold, close stable is invariably damp, and is to be avoided as much as the hot, close, and foul one. Horses changed from a cold to a warm stable are more liable to contract cold than when changed from a warm to a cold one. Pure air is more essential than warmth, and this fact should be especially remembered when the stable is made close and foul to gain the warmth. It is more economical to keep the horse warm with blankets than to prevent the ingress of pure air in order to make the stable warm.