In brief, it is necessary to have a thorough mixing of pure and impure air, so that the combination at different parts of the room may be fairly uniform. To secure these results, the inlets and outlets should be arranged upon principles of ventilation generally accepted by authorities on public health. It seems hardly necessary to say that due attention must be paid to the source from which the introduced air is drawn. If it be taken from foul cellars, or from dirty streets, it may be as impure as that which it is designed to replace. Animal Heat. 223. Animal or Vital Heat. If a thermometer, made for the purpose, be placed for five minutes in the armpit, or under the tongue, it will indicate a temperature of about 98½° F., whether the surrounding atmosphere be warm or cold. This is the natural heat of a healthy person, and in health it rarely varies more than a degree or two. But as the body is constantly losing heat by radiation and conduction, it is evident that if the standard temperature be maintained, a certain amount of heat must be generated within the body to make up for the loss externally. The heat thus produced is known as animal or vital heat. This generation of heat is common to all living organisms. When the mass of the body is large, its heat is readily perceptible to the touch and by its effect upon the thermometer. In mammals and birds the heat-production is more active than in fishes and reptiles, and their temperatures differ in degree even in different species of the same class, according to the special organization of the animal and the general activity of its functions. The temperature of the frog may be 85° F. in June and 41° F. in January. The structure of its tissues is unaltered and their vitality unimpaired by such violent fluctuations. But in man it is necessary not only for health, but even for life, that the temperature should vary only within narrow limits around the mean of 98½° F. We are ignorant of the precise significance of this constancy of temperature in warm-blooded animals, which is as important and peculiar as their average height, Man, undoubtedly, must possess a superior delicacy of organization, hardly revealed by structure, which makes it necessary that he should be shielded from the shocks and jars of varying temperature, that less highly endowed organisms endure with impunity. 224. Sources of Bodily Heat. The heat of the body is generated by the chemical changes, generally spoken of as those of oxidation, which are constantly going on in the tissues. Indeed, whenever protoplasmic materials are being oxidized (the process referred to in sec. 15 as katabolism) heat is being set free. These chemical changes are of various kinds, but the great source of heat is the katabolic process, known as oxidation.