And again (to quote another authority, Dr. Sadler), “All the fluids and secretions of the body are more or less germicidal. The saliva, being alkaline, discourages the growth of germs requiring an acid medium. The normal gastric juice of a healthy stomach is a sure germ-killer. In the early part of digestion, lactic acid is present, and there soon appears the powerful hydrochloric acid, which is a most efficient germicide.... “The living, healthy tissues of the body are all more or less germicidal; that is, they are endowed with certain protective properties against germs and disease. This is true of many of the other special secretions, like those found in the eye and elsewhere in the body, when they are normal. The blood and lymph, the two great circulating fluids of the body, are likewise germicidal. In some conditions of disease, there may be found various substances in the blood which can destroy germs.” THE WHITE CELLS ON GUARD And this definitely brings us to the other kind of inhabitants of the human blood, the leucocytes, or white blood corpuscles,—and so to the germ theory of health, which science is showing to be no less true than the germ theory of disease. In their natural state these cells are transparent, spherical forms of the consistency of jelly drops, which float in the bloodstreams or creep along the inner surface of the vessel. Their function was for a long time not understood; the discovery of the real facts, perhaps the most epoch-making discovery ever made concerning the human body, the world owes to the genius of Metchnikoff, the head of the Pasteur Institute of Paris. These cells are the last reserves of the body in its defense against the assault of disease. Whenever, in spite of all opposition, the hostile germs find access either to the blood or to the tissue, the white cells rush to the spot, and fall upon them and devour them. In their fight against the hordes of evil bacteria that invade the blood, where the battles are waged, the body’s defenders have four main ways of battling. Again we quote from Dr. Ayers: “The blood covers some germs with a sticky paste, and makes them adhere to one another, thereby anchoring them so that they become as helpless as flies on fly paper. The paste comes from the liquid of the blood, the plasma. Another blood-weapon (the ‘lysins’) dissolves the germs as lye does. A third means of defense is the ability of the white blood corpuscles to envelop and digest the living germs. One white cell can digest dozens of germs, but it may mean death to the devouring cells.