CHAPTER III.
HEAT.
IN the last chapter I described the action of the lungs in purifying and supplying oxygen to the blood; but I could not do more than just touch upon the work which that oxygen does in the blood, in producing heat. Before I pass on to consider this, moreover, it will be well to say something about heat in general, for animal heat is subject to general laws which apply throughout nature, and which should be better understood by the majority of people than they are at present.
Our sensations respecting heat and cold are somewhat deceptive if their origin is not comprehended, and they have strengthened the popular belief that heat and cold are two material substances which enter into our bodies; but that this idea is fallacious will presently appear. The nerves of the skin, by which we know whether we feel hot or cold—whether heat is entering or leaving our bodies to a more than ordinary extent, do not enable us exactly to distinguish degrees of
