I may perhaps be allowed to start by drawing a comparison, in order the better to describe what here, in any really comprehensive study of the social question, reveals itself as a powerful, indeed a main, actuating impulse. It must however be borne in mind, that this comparison is intended as a comparison only, used to help out the human understanding and give it the turn of thought needed for picturing what health in the body social implies. Accepting this point of view, then, if one turns to the study of that most complex of all natural organisms, the human organism, it is noticeable, that, running through the whole structure and life of it, there are three systems, working side by side, and each functioning to a certain extent separately and independently of the others. These three neighbour systems may be distinguished as follows: One system, forming a province all to itself in the natural human organism, is that which comprises the life of the nerves and senses. It may be named, after the principal part of the organism where the nerve and sense-life is more or less centred,--the head-system. Second comes what I should like to call the rhythmic system, which, to arrive at any real understanding of man's organisation, must be recognised as forming another branch to itself. This rhythmic system comprises the breathing, the circulation of the blood,--all that finds expression in rhythmic processes within the human organism. The third system, then, must be recognised as comprising all those organs and functions that have to do with actual matter-changes--the metabolic process. These three systems together comprise everything which, duly co-ordinated, keeps the whole human complex in healthy working order.