The anatomical unit of structure is the cell. Cells are the bricks of which the body is built. Some are large, others small, as befits the part which they take in the construction of the body. If the tissue be merely a supporting tissue, connective tissue, cartilage, bone, its cells are uniform in size and small. If it have functions to perform which in some cases are carried out best by small cells, in other cases by large ones, the cells are adapted in size to the work that they have to do. Of the various kinds of wandering cells, some—the bone-forming cells (osteoblasts), for example—are small; others—the bone-eating cells (osteoclasts)—relatively large. Nerve-cells, like telephone exchanges, are large or small according to the size of the area which each supplies. All animals of complex organization, from starfishes and sea-urchins to Man, are inhabited by =motile cells=. In addition to the bricks which enter into the construction of its fabric, each fixed in its place and definitely united to its neighbours, the animal contains leucocytes which wander through its tissue-spaces or float down the streams of lymph or blood. We are disposed to speak of these wanderers as inhabitants of the body, to distinguish them from the elements which enter into the construction of their habitation. It is difficult to avoid the temptation of describing the body as a habitation. Allegorical as Aristotle’s distinction between body and soul—between the habitation and that which inhabits—may seem, when contrasted with the exact language of modern science, it would save many a periphrasis if we might still use the monosyllable “soul.” The fixed tissues constitute a unity, bound together by nerves. The work done by glands and muscles is done in response to directions conveyed by nerves. It is impossible to say where the control of the nerves ceases—to point out any fixed tissue which is not co-ordinated with other tissues, nor susceptible to the influence of the environment as impressed upon the central nervous system, through the medium of sense-organs. The fixed tissues constitute a habitation for the “soul.” They share in a common life. The wandering cells are as independent of control as the parasites which occasionally find entrance into the body. Each must have a soul of its own in Aristotle’s sense. Like parasites, they carry on all the business of nutrition, respiration, cell division, without reference to the needs of the fixed tissues. They take what they require from the lymph as it leaves the intestines loaded with the products of digestion; they take it from the lymph in the tissue-spaces; they take it from the blood. When nutriment or oxygen runs short, they do not share the privations of the fixed tissues. Only indirectly is their well-being affected by that of the body as a whole; only accidentally is the death of the body the occasion of their death. The same might be said of such parasites as