Thirst, like appetite for food, is intended to direct us when and in what quantity we ought to drink; and so long as we lead a life of ordinary health and activity, and confine ourselves to the fluids with which nature provides us, there is little chance of our going far wrong by listening to its calls. But when we become indolent and dyspeptic, or resort to the use of fermented and stimulating liquors, which excite a thirst not recognised by Nature, the principle ceases to apply. At present, however, my observations refer entirely to such simple drinks as water, and to the state of health; and I shall touch upon other liquids when treating of diet in a subsequent part of the volume. Many persons without experiencing any real thirst, habitually indulge in potations of water or beer at all hours of the day, and to an extraordinary extent, and feel unhappy when suddenly restricted in the indulgence. But this temporary discomfort ought not to be considered as indicating that these potations are really necessary, because the same result happens in the analogous instances of smoking or snuffing. All three are abuses and perversions of Nature, and the uneasiness attending the sudden cessation of the beer or water drinking is no more a proof of either fluid being required, than that consequent on giving up cigar smoking is an indication that Nature designed the lungs for the reception of the impure effluvia of the tobacco leaf instead of the fresh breezes of heaven. 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. Having seen that a regular supply of nourishment is carefully insured by the constantly returning impulses of Appetite, we come next to examine the mode in which the food is prepared for becoming a constituent part of the animal machine, and endowed with the properties of life. The first important step in the complicated process of digestion, is that by which the food, after being received into the mouth, is mixed with the saliva and broken down till it becomes of a uniform pulpy consistence, fit for being easily swallowed and acted upon by the gastric juice on its arrival in the stomach. The term _mastication_ or _chewing_ is used to denote this operation; and the chief instruments by which it is performed are the teeth, the jaws, the muscles which move the jaws, the tongue, and the salivary glands. On each of these we shall offer a few observations. [Illustration]