The physiologist, if he is to feel sure of his ground, needs to know the minute anatomy as well as the naked eye anatomy of the body. But what is there that he does not need to know? He must be chemist, physicist, biologist, pathologist, and expert in various other branches of science. Microscopic anatomy, or histology, as it is commonly termed, will be called upon in this book only when it has evidence to give which bears directly on physiological problems. We have dwelt at some length upon the cell theory because the physiologist needs starting-points. He needs to have in his mind a conception of the fundamental structure of the body. Protoplasm is the material which lives. We begin with protoplasm albeit our conception of protoplasm is so difficult to formulate that we are obliged to admit that in using the term we are almost guilty of playing with words. Protoplasm is the most living substance. The substance which is most alive always presents itself to us as an imperfectly transparent, viscous material, which proves on analysis to contain a large quantity of certain proteins mixed with various organic and inorganic compounds. Protoplasm is organized into, or distributed amongst, cells, which in any given tissue present a fairly uniform size. What determines the size of cells? Speaking generally, cells are small—say about 0·01 millimetre in diameter. In early stages of growth, cell division occurs as soon as the cell attains to something like this size. It would seem that when nutriment is abundant cells add to their protoplasm more than they lose. Having attained certain dimensions at which the conditions most satisfactory for cell life reach their limit, cell division occurs. The big drop falls into two smaller drops, each of which grows more rapidly than the big one was growing at the time when it began to divide. But if there be an optimum size for nutritive purposes, this limit is suspended in many cases, and for various reasons. Take the ovum itself as an example. It is vastly bigger than the cells into which it divides. The yolk of a hen’s egg is, when first formed, a single cell. By the time the egg is laid cell division has already set in. In the embryo there are cells which surpass the average dimensions—the unexplained “giant cells” which appear in the liver as soon as it can be recognized as such (_cf._ p. 65). These disappear from the liver, but are for a time evident in the spleen. The large cells found in the marrow of bone, some with a great single nucleus, others containing a bunch of separate nuclei, also show that there is no fixed limit of size. It is generally considered that the giant cells of marrow—or, at any rate, those which are multinucleated—are leucocytes which are engaged in scooping out the bone; consuming the hard tissue on the inner surface of the hollow cylinder in order that, by deposition of new material on the outside of the cylinder, the size of the whole