=Phagocytosis.=--All leukocytes have the power of shifting their location. The lymphocytes, so called, being the youngest of the white corpuscles, show it less than the older forms. The eosinophile cells are less able to manifest the peculiar activities of the other forms. It is particularly the mononuclear and polynuclear corpuscles which are endowed with most pronounced activity. These have the power, like the ameba among the lowest forms of life, to not only spread themselves around inert bodies, like granules of carmine or other particles used for experiment, or the particles of coal-dust found in certain conditions in the human body, but they also have the power to englobe many living organisms, for the main part vegetable (bacteria). Under the microscope it is possible to see living bacilli, performing active movements, although enclosed in the nutritive vacuoles of the leukocytes, in some of the lower animals. This _ameboid_ power possessed by these cells of thus attacking and disposing of foreign bodies or irritants has been demonstrated and proved, especially by Metchnikoff, and has been called by him _phagocytosis_. His views were for a long time disputed, and are perhaps not yet absolutely and generally accepted. Nevertheless, they fulfil every demand made upon them for explanation, and are susceptible of such demonstration under the microscope that we now have practically a new and apparently a correct theory of the inflammatory process. (See Chapter III.) Any cell which has this property is known as a _phagocyte_. It is shared by some of the leukocytes with certain other cells to be spoken of later (wandering tissue cells). Cells which possess this power do not attract all microbes indiscriminately, and it is often the case that the leukocytes of an animal peculiarly susceptible to a certain kind of bacteria do not attract them at all, even though they are directly in contact. It is plausible that an explanation of the peculiar susceptibility of certain animals to certain diseases is furnished by this fact (Fig. 3). [Illustration: FIG. 3 Phagocytosis in anthrax pustule. (Gaylord.)]