Naturally the mind of man was first educated to observe external objects and forces in their effects upon himself, and the external still continues to engross his attention as if he were a child in a kindergarten. Fascinated by the Without, he ignores the Within. But, marvel of marvels, Disease (which when looked at with discerning eyes is seen to be an angel in disguise) comes to enlighten him concerning the world within. Disease gradually acquaints him with the fact that there are within him organs and functions corresponding to the objects and forces in the world without,--servitors in fact which must not be ignored under penalty of transforming them into foes to his well-being. Disease makes him aware that by ignoring the claims of his inner relations he has been converting his very food, juices and gases into insidious and formidable poisons, which perforce he absorbs into his blood and tissues and circulates throughout his entire system. Thus does the disguised angel admonish the ignorant that the rights of the inner world must not be ignored--that one's duties thereto cannot be neglected without disastrous consequences. Thus does Pathology, which is really Physiology reversed, become the self-revealer _par excellence_. Through digestion and assimilation the physiological process takes up the food, juices and gases, to support and augment the life of man. The pathological process, on the contrary, because the conditions for nutrition are ignored, reverses the upbuilding processes; and the organs of life wither, waste and weaken, until life goes out like fire unfed. Man has been slowly learning to take sanitary measures in reference to everything that contributes to comfort in his surroundings, and hygienic measures in reference to everything conducive to stability in his health. Through ages he has learned, by experience and experiment, of the changes that inevitably occur in such perishable nutritive substances as water, milk, meats, vegetables, fruits, etc., if they be left uncared for; and he has been led thus to the inference of the law of decomposition--or putrefactive and fermentative changes. Idle substances, like idle minds, have decomposition and the devil for companions. Substances confined in containers open to the air--ponds, cesspools, etc.--are every-day object lessons to man of the fact that the chemical changes they undergo furnish the conditions for breeding bacterial poisons, and that these poisons are a dread menace to animal life.