Nutrition not a simple process. (1) Need of subsidiary organs for the various stages of alteration, _e.g._, of bread into blood, of that into bone, etc. (2) Need also of organs for excreting the non-utilizable portions of the food, _e.g._, much vegetable matter is superfluous. (3) Need of organs of a third kind, for distributing the pabulum through the body. Chapter XI Nutrition analysed into the stages of application (_prosthesis_), adhesion (_prosphysis_), and assimilation. The stages illustrated by certain pathological conditions. Different shades of meaning of the term _nutriment_. Chapter XII The two chief medico-philosophical schools--Atomist and Vitalist. Hippocrates an adherent of the latter school--his doctrine of an original principle or "nature" in every living thing (doctrine of the unity of the organism). Chapter XIII Failure of Asclepiades to understand the functions of kidneys and ureters. His hypothesis of vaporization of imbibed fluids is here refuted. A demonstration of urinary secretion in the living animal; the forethought and artistic skill of Nature vindicated. Refutation also of Asclepiades's disbelief in the special selective action of purgative drugs. Chapter XIV While Asclepiades denies _in toto_ the obvious fact of specific attraction, Epicurus grants the fact, although his attempt to explain it by the atomic hypothesis breaks down. Refutation of the Epicurean theory of magnetic attraction. Instances of specific attraction of thorns and animal poisons by medicaments, of moisture by corn, etc. Chapter XV It now being granted that the urine is secreted by the kidneys, the _rationale_ of this secretion is enquired into. The kidneys are not mechanical filters, but are by virtue of their _nature_ possessed of a specific faculty of attraction. Chapter XVI Erasistratus, again, by his favourite principle of _horror vacui_ could never explain the secretion of urine by the kidneys. While, however, he acknowledged that the kidneys do secrete urine, he makes no attempt to explain this; he ignores, but does not attempt to refute, the Hippocratic doctrine of specific _attraction_. "Servile" position taken up by Asclepiades and Erasistratus in regard to this function of urinary secretion. Chapter XVII Three other attempts (by adherents of the Erasistratean school and by Lycus of Macedonia) to explain how the kidneys come to separate out urine from the blood. All these ignore the obvious principle of attraction. BOOK II Chapter I