119. This instinctive feeling announces to the individual the necessity of introducing a certain quantity of liquid into the system, in order to repair the waste which the body has sustained in the exercise of its functions; or to impart a due degree of solubility to the aliments which have been taken. We accordingly find that excessive perspiration increases the demand, and dry food is followed by the same effect. With the history of morbid thirst we have at present nothing to do. The sensation is usually referred to the throat and fauces, as that of hunger is to the stomach; and yet the intensity of this feeling does not bear any relation to the dryness of these parts: for in some cases, where the tongue, to its very root, is covered with a thick and dry crust, there is little thirst; while, on the other hand, it is frequently intolerable at the very time the mouth is surcharged with a preternatural quantity of saliva: like hunger, I apprehend it must be referred to a particular condition of the nerves wisely ordained to express the wants of the system.

Dr. Gardner has remarked that, in the case of a person who had cut through the oesophagus, several buckets-full of water were swallowed daily, and discharged through the wound, without quenching the thirst, which was afterwards found to abate by the injection of diluted spirit into the stomach. Were I to consult my sensations, the seat of thirst would be referred to the upper orifice of the stomach. The desire for drink after long speaking has some analogy to thirst, but must not be confounded with it. The influence of salted food in exciting this sensation is to be explained upon the simple principle that saline bodies require dilution to mitigate their acrimony.

120. Thirst is certainly under the control of habit: those who indulge in frequent potation are rendered thirsty by its privation. There are some persons who have never experienced the sensation, and who only drink from a sort of sympathy, but who could live a long time without thinking of it, or without suffering from the want of it. I have a lady, of fifty years of age, at this time under my care, who has declared that she is perfectly unacquainted with the nature of the sensation. Sauvage relates two similar instances that occurred to himself; and Blumenbach, also, quotes several examples of the same idiosyncrasy.

121. The sensations of hunger and thirst generally appear to be incompatible with each other: when the stomach requires food, there is rarely any inclination to drink; and when thirst rages, the idea of solid aliment disgusts us. So, again, those circumstances which tend to destroy appetite may even excite thirst, such as the passions of the mind, etc.

122. When the healthy system is in a condition to require food, besides the local sensation of hunger, there are certain general phenomena which deserve notice; - a universal lassitude of the body is experienced; there is also a sensation of pressure, or drawing down, in the epigastric region; the diameter of the intestines becomes diminished, and their peristaltic motion being at the same time increased, portions of contained air are successively displaced, which give rise to gurgling sounds. There is, besides, an alteration in the situation of some of the abdominal viscera; they are less capable of sustaining pressure, and they receive a less quantity of blood. M. Majendie also supposes, that when the stomach is empty, all the reservoirs contained in the abdomen are more easily distended by the matters which remain some time in them; and he believes that this is the principal reason why bile then accumulates in the gall-bladder. As soon as a certain quantity of food has entered the stomach, the general feeling of lassitude gives place to that of renewed force, and this usually occurs more rapidly after the ingestion of liquid than of solid aliment; which is sufficient to prove that the phenomenon results from a local action upon the nerves of the stomach, since in neither case is it possible that any nutritive principle can have been so rapidly transferred to the system.

123. As soon as digestion commences, the blood flows with increased force to the organs destined for its completion; whence, in delicate persons, the operation is frequently attended with a diminution in the power of the senses, and a slight shiver is even experienced: the skin becomes contracted, and the insensible perspiration is diminished. As the process, however, proceeds, a re-action takes place; and, after it is completed, the perspiration becomes free, and often abundant. When the chyle enters the blood, the body becomes enlivened, and the stomach and small intestines, having been liberated from their burden, oppose no obstacle to the free indulgence of that desire for activity, which nature has thus instinctively excited for our benefit. Then it is that animals are roused from that repose into which they had subsided during the earlier stages of digestion, and betake themselves to action; then it is that civilized man feels an aptness for exertion, although he mistakes the nature and object of the impulse, and, as Dr. Prout justly observes, is inclined to regard it as nothing more than a healthy sensation, by which he is summoned to that occupation to which inclination or duty may prompt him.

Thus, instead of being bodily active, the studious man receives it as a summons to mental exertion; the indolent man, perhaps, merely to sit up and enjoy himself; the libertine to commence his libations; and the votary of fashion to attend the crowded circles of gaiety and dissipation: in short, this feeling of renovated energy is used, or abused, in a thousand ways by different individuals, without their ever dreaming that bodily exercise, and that alone, is implied by it. The result of which is, that imperfect assimilation, and all its train of consequences, take place.

124. Some difference of opinion has existed with regard to the utility or mischief of exercise immediately after eating; but in this question, as in most others of the like nature, the truth will be found to lie between the extremes. Those who, from confounding the effects of gentle with those of exhausting exercise, maintain the necessity of rest for the perfect performance of the digestive process, appeal to the experiment of Sir Busick Harwood, the mere relation of which will be sufficient to negative the inference which they would deduce from its result. The Downing Professor took two pointers, equally hungry, and equally well fed; the one he suffered to lie quiet after his meal, the other he kept for above two hours in constant exercise. On returning home he had them both killed. In the stomach of the dog that had remained quiet and asleep, all the food was found chymified; but in the stomach of the other dog, the process of digestion had scarcely commenced. Exercise, let it be remembered, must be measured in relation to the strength and habits of the individual: we have daily experience to prove that the husbandman may return to his daily labour, and the schoolboy to his gambols, immediately after a frugal meal, without inconvenience or injury; but the same degree of exercise to a person of sedentary habits, or of weak stamina, would probably arrest and subvert the whole process of digestion.

The influence of habit, in rendering exercise salutary or injurious, is shown in a variety of instances: a person who would suffer from the slightest exertion after dinner, will undertake a fatiguing labour after breakfast, however solid and copious that meal may have been. If we assent to the proposition of the Cambridge Professor, we must in consistency acknowledge, that exercise, before a meal, is at least as injurious as he would lead us to suppose it is after a repast: for if the valetudinarian take his dinner in a state of fatigue, he will assuredly experience some impediment in its digestion; but are we to argue that, on this account, exercise is neither to precede nor follow a meal? We may as well, without further discussion, subscribe to the opinion of Hierony-mus Cardanus, who, insisting upon the advantages of perfect rest, observes, that trees live longer than animals, because they never stir from their places.