THE APPETITES OF HUNGER AND THIRST. Hunger and Thirst, what they are--Generally referred to the stomach and throat, but perceived by the brain--Proofs and illustrations--Exciting causes of hunger--Common theories unsatisfactory--Hunger sympathetic of the state of the body as well as of the stomach--Uses of appetite--Relation between waste and appetite--Its practical importance--Consequences of overlooking it illustrated by analogy of the whole animal kingdom--Disease from acting in opposition to this relation--Effect of exercise on appetite explained--Diseased appetite--Thirst--Seat of Thirst--Circumstances in which it is most felt--Extraordinary effects of injection of water into the veins in cholera--Uses of thirst, and rules for gratifying it, 11–39 CHAPTER III. MASTICATION, INSALIVATION, AND DEGLUTITION. Mastication--The teeth--Teeth, being adapted to the kind of food, vary at different ages and in different animals--Teeth classed and described--Vitality of teeth and its advantages--Causes of disease in teeth--Means of protection--Insalivation and its uses--Gratification of taste in mastication--Deglutition, 40–57 CHAPTER IV. ORGANS OF DIGESTION--THE STOMACH--THE GASTRIC JUICE. Surprising power of digestion--Variety of sources of food--All structures, however different, formed from the same blood--General view of digestion, chymification, chylification, sanguification, nutrition--The stomach in polypes, in quadrupeds, and in man--Its position, size, and complexity, in different animals--Its structure; its peritoneal, muscular, and villous coats; and uses of each--Its nerves and bloodvessels; their nature, origins, and uses--The former the medium of communication between the brain and stomach--Their relation to undigested food--Animals not conscious of what goes on in the stomach--Advantages of this arrangement--The gastric juice the grand agent in digestion--Its origin and nature--Singular case of gunshot wound making a permanent opening into the stomach--Instructive experiments made by Dr Beaumont--Important results, 58–108 CHAPTER V. THEORY AND LAWS OF DIGESTION.