The body is a machine constructed for the purpose of working. The kinds of work which the body of a man or woman does are many. Every act of perception or sensation, is an act of work; so is every thought, every emotion, every volition. The action of the heart or lungs in the circulation and respiration, the evolution of the animal heat, the various functions of secretion and excretion, digestion, motion, speech, etc., are all so many kinds of work. Now as regards work, it is well known that for its due performance force is required, and it is equally well known that for the development of force, matter that can be metamorphosed is necessary. The engine may be perfect, the water may be in the boiler, but unless there be force in the form of heat there will be no steam; and there will be no heat unless there be fuel in a state of combustion. The human body differs from any other machine in the fact that it uses its fuel in great part indirectly; only in fact after it has been assimilated and converted into tissues of various kinds. Thus when a muscle contracts, it is the muscle itself which is consumed; when a thought is conceived it is the brain which provides the force; when an emotion is experienced, it is again the brain which is decomposed. The body, therefore, lives by the death of its own substance. It is true, some kinds of food such as alcohol, tea and coffee, and perhaps some others do not require to go farther than the blood to be burned, but these are mainly heat-producing, and not tissue-producing substances. But whether matter be consumed directly or indirectly, all bodily force results from its decomposition, and without this destruction of matter the body would be absolutely incapable of a single functional action of any kind whatever, and its temperature like that of the so-called cold-blooded animals would be that of the surrounding medium, the atmosphere.