The process by which the white cells fight for us may be watched in the transparent tissue of a frog’s foot or the wing of a bat. If a few disease germs are introduced into this tissue, the white cells may be seen to accumulate on the wall of the blood vessel just opposite where the germs have entered. “Each cell begins to push out a minute thread of its tissue,” writes Dr. Kellogg, in describing the process, “thrusting it through the wall of its own blood vessel. Little by little the farther end of this delicate filament which has been pushed through the wall grows larger and larger, while the portion of the cell within the vessel lessens, and after a little time each cell is found outside the vessel, and yet no openings are left behind. Just how they accomplish this without leaving a gap behind them is one of the mysteries for which Science has for many years in vain sought a solution. The vessel wall remains as perfect as it was before. Apparently, each cell has made a minute opening and has then tucked itself through, as one might tuck a pocket handkerchief through a ring, invisibly closing up behind itself the opening made. Once outside the vessel, these wonderful body-defenders, moving here and there, quickly discover the germs and proceed at once to swallow them. If the germs are few in number, they may be in this way destroyed, for the white cells not only swallow germs, but digest them. If the number is very great, however, the cells sacrifice themselves in the effort to destroy the germs, taking in a larger number than they are able to digest and destroy. When this occurs, the germs continue to grow; more white cells make their way out of the blood vessels, and a fierce and often long-continued battle is waged between the living blood cells and the invading germs.” Now, it must be understood that this description is not the product of any one’s imagination, but is a definitely established fact which has been studied by scientists all over the world. Because of the importance of the discovery, and of the new views of health to which it leads, we have placed a picture of this “battle of the blood” at the front of this book. It shows the leucocytes of the human body in conflict with the germs of influenza: the black dots being the germs, and the larger grayish bodies the leucocytes. We have chosen a photograph rather than a drawing, so that the reader may realize that he is seeing something which actually has existence. We request him to study the picture and fix it upon his mind, for it is not too much to say that from it is derived every principle of health which is set forth in the course of this book. THE PROBLEM OF HEALTH