Continued thirst, it is well known, is much more intolerable than continued hunger. The mass of circulating fluid in the body is very great, and, as the various excretions consist chiefly of fluid matter, it necessarily happens that when these have been eliminated for a considerable time without any liquid being received into the system, the proportion of solid matter in the body becomes unduly large. The blood, consequently, becomes thicker and changed in quality, and much more irritating than it is in its natural state. The craving of thirst is thus generally rendered more urgent and overpowering than that of hunger.
In Asiatic Cholera, the watery portion of the blood, on which its fluidity depends, is drained off with frightful rapidity; and the result is, in the first place, an almost complete stoppage of the circulation, and, in the second, a constant craving for drink to supply the place of the lost serum, which consists chiefly of water holding some of the alkaline salts in solution. This circumstance explains in some degree the extraordinary effects which have been produced, even in the worst stages of the disease when life seemed almost extinct, by injecting large quantities of saline solutions into the veins. Patients apparently on the verge of existence, cold, pulseless, and inanimate, have in the course of a few minutes been enabled by this means to sit up in bed, and to exhibit all the signs of restored strength and health. The effect, it is true, was rarely permanent, but for the time it was so wonderful as often to look like restoration from the dead.[12]
Fluids taken into the stomach, it is proper to observe, are not subjected to the slow process of digestion, but are absorbed directly into the system; so that, when we take a moderate draught, the whole of it is imbibed from the stomach in a very few minutes. Keeping in view this fact, and the above striking illustration of the influence of the condition of the blood upon the body at large, it becomes easy to conceive why, in a state of exhaustion from abstinence, drink should be more speedily restorative and refreshing than food.
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.
The TEETH vary a good deal according to the kind of food on which the animal is destined to live; but in man and the higher orders of animals they may be divided into three distinct groups:—1st, The incisor or cutting teeth, being the eight broad and flat teeth with a sharp cutting edge seen in front of the upper and lower jaws, and marked I in the subjoined wood-cut, which represents one-half of the lower jaw, and consequently only one-fourth of the whole number of teeth. Thus we find only two incisors marked in the wood-cut, although there are eight of them in all, viz. two more are on the other side of the lower jaw, and four corresponding ones in the upper. 2d, The cuspidati, canine, or dog teeth, being the sharp-pointed roundish-bodied teeth, four in number, one, C, in contact with each of the outer incisor teeth, and called canine from being large in the dog and carnivorous animals, and used by them for the purpose of seizing and tearing their food; and, 3d, The molares or grinders, B G, twenty in number situated at the back part of the jaw, and so called from their office being to grind or bruise the food subjected to their action.[13] The term grinders, however, is sometimes restricted to the three back teeth on each side, marked G, and seen to have double roots and a broad grinding surface; and the two, B, intervening between them and the cuspidati are styled bicuspidati or double-speared, from bearing a greater resemblance to a double-headed canine tooth than to the other grinders.