Again, in the case of hunger, the introduction of innutritions matters, as earth, or sawdust, will somewhat relieve the urgent sensations in extreme cases, as will also the use of tobacco by smokers, or much mental occupation, though this is rather illustrative of the lessening of the consciousness of the ingoing impulses by diverting the attention from them. But hunger, like thirst, may be mitigated by injections into the intestines or the blood. It is, therefore, clear that, while in the case of hunger and thirst there is a local expression of a need, a peculiar sensation, more pronounced in certain parts (the fauces in the case of thirst, the stomach in that of hunger), yet these may be appeased from within through the medium of the blood, as well as from without by the introduction of food or water, as the case may be.
Up to the present we have assumed that the changes wrought in the food in the alimentary tract were identical with those produced by the digestive ferments as obtained by extracts of the organs naturally furnishing them. But for many reasons it seems probable that artificial digestion can not be regarded as parallel with the natural processes except in a very general way. When we take into account the absence of muscular movements, regulated according to no rigid principles, but varying with innumerable circumstances in all probability, the absence of the influence of the nervous system determining the variations in the quantity and composition of the outflow of the secretions; the changes in the rate of so-called absorption, which doubtless influences also the act of the secretion of the juices—by these and a host of other considerations we are led to hesitate before we commit ourselves too unreservedly to the belief that the processes of natural digestion can be exactly imitated in the laboratory.
What is it which, enables one man to digest habitually what may be almost a poison to another? How is it that each one can dispose readily of a food at one time that at another is quite indigestible? To reply that, in the one case, the digestive fluids are poured out and in the other not, is to go little below the surface, for one asks the reason of this, if it be a fact, as it no doubt is. When we look further into the peculiarities of digestion, etc., we recognize the influence of race as such, and in the race and the individual that obtrusive though ill-understood fact—the force of habit, operative here as elsewhere. And there can be little doubt that the habits of a people, as to food eaten and digestive peculiarities established, become organized, fixed, and transmitted to posterity.
It is probably in this way that, in the course of the evolution of the various groups of animals, they have come to vary so much in their choice of food and in their digestive processes, did we but know them thoroughly as they are; for to assume that even the