There is a parallel to this in the chemical side of life, where we find some phenomena peculiar to the living elements, and others, like digestion, going on in the living body, but outside the cells. Taking the movements in the natural order—that is, proceeding from the simpler to the more complex—the first to be considered is undoubtedly that of the leucocytes, or general scavengers of the tissues. The body consists, so far as we have defined its anatomy, of three layers of cells, and its shape is that of a tube with hollow walls. (See Diagram 6.) Within the cavity of the body are various organs, such as the muscles, which are formed from the middle layer; and its space is largely reduced by glands, lungs, and other ramifications of the inner layer which forms the alimentary canal. These organs hang more or less freely in the body cavity, slung to its walls by enveloping sheets of connective tissue, the whole being bathed in lymph. Now, in such an arrangement the products of wear and tear must accumulate. Cells here and there die for various reasons, and pieces of cells become detached even in adult animals. The interior of a bone is always being eaten away to decrease its weight, or in order that it may be replaced by fresh bone of a closer texture, and in young animals and embryos there are many structures which, useful for a time, have eventually to be removed; as an instance, we may quote the tadpole’s tail. In fact, if the tissues were left to themselves, the body would soon be choked with débris, and to avoid this it is supplied with an army of scavengers, the leucocytes. The leucocytes are detached cells which owe their origin to the middle layer. In size they are, of course, very small, quite invisible to the naked eye. In appearance they resemble unicellular organisms of the amœba type, which we have had occasion to mention several times already (Essay II., Section II.; Essay III., Section I., Diagram 1). They are of several different varieties, some being larger and more active than others; but they all wander about in the lymph and blood like independent animals, creeping in and out between the cells of the organs, and devouring any foreign matter they come across. They sometimes multiply, like independent animals, by division, especially in the presence of inflammation, or when they have much work to do, and a rapid increase in their numbers is needed; and they have been induced to live, and feed, and multiply, outside the body (in which case they must be considered to have become independent organisms), thanks to the careful attentions of the experimenter.