A living body, considered as a physico-chemical mechanism, has some very remarkable properties, which, so far, no machine of human construction can imitate. The physical parts of the mechanism, such as the heart’s action in pumping the blood, and the working of muscles and bones, are less remarkable than the chemical portions, but have at any rate the merit of seldom going seriously out of order. The heart has to work day and night throughout the whole of a man’s life—say, seventy years. Repairs, if any are needed, have to be effected while it is working. An ordinary healthy man is much less often ill than the best of motorcars, in spite of the fact that his engine never gets a rest. The physics of the human body is excellent, but is less complex and interesting than its chemistry.