Interaction between two bodies; the stronger masters the weaker; a deleterious drug masters the forces of the body, whereas food is mastered by them; this mastery is an _alteration_, and the amount of alteration varies with the different organs; thus a partial alteration is effected in mouth by saliva, but much greater in stomach, where not only gastric juice, but also bile, pneuma, innate heat (_i.e._ oxidation?), and other powerful factors are brought to bear on it; need of considerable alteration in stomach as a transition-stage between food and blood; appearance of faeces in intestine another proof of great alteration effected in stomach. Asclepiades's denial of real qualitative change in stomach rebutted. Erasistratus's denial that digestion in any way resembles a _boiling_ process comes from his taking words too literally. Chapter VIII Erasistratus denies that the stomach exerts any pull in the act of swallowing. That he is wrong, however, is proved by the anatomical structure of the stomach--its inner coat with longitudinal fibres obviously acts as a _vis a fronte_ (attraction), whilst its outer coat exercises through the contraction of its circular fibres a _vis a tergo_ (propulsion); the latter also comes into play in vomiting. The stomach uses the oesophagus as a kind of hand, to draw in its food with. The functions of the two coats proved also by vivisection. Swallowing cannot be attributed merely to the force of gravity. Chapter IX These four faculties which subserve nutrition are thus apparent in many different parts of the body. Chapter X Need for elaborating the statements of the ancient physicians. Superiority of Ancients to Moderns. This state of affairs can only be rectified by a really efficient education of youth. The chief requisites of such an education. Chapter XI For the sake of the few who realty wish truth, the argument will be continued. A third kind of fibre--the _oblique_--subserves _retention_; the way in which this fibre is disposed in different coats. Chapter XII The factor which brings the expulsive faculty into action is essentially a condition of the organ or its contents which is the reverse of that which determined attraction. Analogy between abortion and normal parturition. Whatever produces _discomfort_ must be expelled. That discomfort also determines expulsion of contents from gall-bladder is not so evident as in the case of stomach, uterus, urinary bladder, etc., but can be logically demonstrated. Chapter XIII