The common impurities found in _stable air_ are carbonic and ammonia gas, moisture charged with injurious matter and dust from the floor and bodies of the animals. As a rule, the more crowded and filthy the stable, the more impurities there are in the air. If any of the animals are affected with an infectious disease, such as tuberculosis or glanders, the moisture and dust may act as carriers of the disease-producing germs. Infectious diseases spread rapidly in crowded, poorly ventilated stables. The two factors responsible for this rapid spread of disease are the lowered vitality of the animal, due to breathing the vitiated air, and the greater opportunity for infection, because of the comparatively large number of bacteria present in the air. _The purpose of stable ventilation_ is to replace the stable air with purer air. The frequency with which the air in the stable should be changed depends on the cubic feet of air space provided for each animal, and the sanitary conditions present. The principal factor in stable ventilation is the force of the wind. In cold weather it is very difficult to properly ventilate a crowded stable without too much loss of animal heat and creating draughts. For practical purposes, the _need of ventilation_ in a stable can be determined by the odor of the air, the amount of moisture present and the temperature. It is impossible to keep the air within the stable as pure as the atmosphere outside. All dangers from injury by breathing impure air, or by draughts can be eliminated by proper stable construction, attention to the ventilation and keeping the quarters clean. [Illustration: FIG. 22.--A case of catarrhal cold.] CATARRH (COLD IN THE HEAD).--Catarrh is an inflammation of the mucous membrane lining the nasal cavities that usually extends to the membrane lining of the sinuses of the head. It may be acute or chronic. The inflammation very often extends to the pharynx and larynx. Cold in the head is more common in the horse than in any of the other animals (Fig. 22). _The most common causes_ of "colds" are standing or lying in a draught, becoming wet, and exposure to the cold. "Colds" are common during cold, changeable weather. Horses that are accustomed to warm stables, are very apt to take "cold" if changed to a cold stable and not protected with a blanket. Most animals are not affected by the cold weather if given dry quarters and a dry bed. Irritation to the mucous membrane by dust, gases and germs is a common cause. Influenza and colt distemper are characterized by an inflammation of the respiratory mucous membranes. In the horse, chronic catarrh is commonly caused by diseased teeth, and injuries to the wall of the maxillary sinus. In sheep, the larvae of the bot-fly may cause catarrh.