A natural question arising out of the study of leucocytes is, What becomes of them? Particles of soot and similar refuse can hardly be considered nutritious, or even digestible, food, and one is rather drawn to the conclusion that the leucocyte performs its functions for the good of the body at large, not of itself, and that when its work is done it must die. Many leucocytes, probably, loaded with unconsidered and undesirable trifles, cast themselves into the alimentary canal, and are got rid of with the useless portions of the food; but they do not always have the luck or energy to get to a natural outlet. An unpleasantly familiar phenomenon is the boil. Here we have some irritating substance under the skin setting up inflammation, and leucocytes swarm up to remove the cause of the trouble. Before, however, this is done, many have perished in the fray, and they have collected in numbers to the formation of what is commonly known as pus, or matter. Their dispersal into the body is now neither easy nor desirable, and the surgeon usually lets them escape from the surface by a touch of the lancet. [Illustration: DIAGRAM 31. A, Eosinophile leucocyte; B, bacterium; C, leucocytes killing bacterium with their enzyme; D, leucocytes leaving bacterium dead; E, hyaline leucocyte devouring dead bacterium.] Such, then, is very briefly the story of the leucocyte, neglecting such problems as the differences between those found in the blood, called white corpuscles to distinguish them from the red corpuscles, with which they have no sort of connection; those found in the lymph, called lymphocytes to distinguish them from those found in the blood; those caught in the act of devouring bone, called osteoclasts; and those found with bacteria inside them, therefore known as phagocytes; and without speculating on how long an individual lives, and whether the different varieties differ in origin or are merely at progressive stages of development. The study of leucocytes is one of the most fascinating in physiology, but we have many other things calling for our attention, and we have said enough about the part they play in the life of the body to justify our passing on to consider another essential movement. IV. Next in natural order for consideration come the movements of the alimentary canal. So far we have considered this structure as a chemical laboratory, a tube consisting of a single layer of cells which secrete ferments into the lumen, where digestion takes place, and then absorb the products, and we have not yet accounted for the food travelling along the tube, without which its functions, as described in the earlier part of the book, could not be performed. That the passage of the food is not due to gravitation is obvious from the many directions of the tube’s coils—not to quote the old instance of a horse drinking, in which case the liquid first travels upwards. One must therefore conclude in favour of some muscular method of propulsion.