agents, they must be always more or less pernicious, when made habitual articles of diet. - A third class of substances remains to be noticed, such as are not in themselves nutritive, but which are added to our food for the purpose of giving flavour to it, stiled condiments. These are very numerous, and derived from very different sources, but they may be all reduced to the two heads of salts and spices. Their selection appears to depend upon very singular habits or even caprices, so that those substances which are the most grateful to certain individuals and classes of people, are the most disagreeable, or even nauseous, to others. It may be laid down as a general principle, that such articles as are, in the first instance, disagreeable to the palate, are those for which we afterwards acquire the strongest partiality, and which even become necessary for our comfort; whereas the frequent repetition of flavours that are originally grateful, is very apt to produce a sense of satiety, or even of disgust. The examples of tobacco, garlic, and assafoetida, on the one hand, and of such substances as possess simple sweetness on the other, may be adduced in proof of this position. There is such a very general relish for sapid food, among all descriptions of people, and in all states of civilization, as to induce us to suppose that besides the mere gratification of the palate, some useful purpose must be served by it, and that it must contribute, in some important manner, to the digestion of our food. This peculiar kind of taste, with a few exceptions, does not seem to exist among the inferior animals, who generally prefer the species of food, which is best adapted to their organs, in a simple state. Perhaps, this may be, in some measure, explained by the consideration that man differs from other animals in his capacity of existing in all climates and in all situations, and that, in order to enable him to do this, it was necessary that he should be omnivorous, as it is termed, that is, able to digest any substance, which affords the elements of nutrition. But, as in the process of chymification, it appears that all the aliment received into the stomach must be reduced to a mass nearly uniform in its cónstitution, it is reasonable to suppose that some assisting or correcting substances may be requisite to reduce the various species of aliment to one uniform standard. Vegetable substances, for example, when reduced to a soft pulp, and macerated at the temperature of the stomach, would probably have a tendency to the acetous fermentation, which we may presume will be corrected by the addition of aromatics and spices, whereas a mass of animal matter may be prevented from degenerating into the putrid state by salts or acids. Both these articles seem to be originally agreeable to the palate; and as a general rule, we shall find that they are naturally produced in the regions where they are the most required. The different species of brute animals exist in those countries alone where there is a supply of food adapted to their diges tive organs, and as they are guided in their choice of the articles of diet by instinct, they do not require the aid of these correctives. There is, however, one remarkable exception to this general rule, in the relish which many animals of the higher orders seem to have for salt. Besides the various kinds of fish that inhabit the sea, and the birds that feed upon marine animals, we find that many quadrupeds and land birds clearly indicate their fondness for salt, and we have daily examples presented to us of its salutary operation, when either incidentally or intentionally mixed with the food of animals. Many singular instances are mentioned of the extraordinary efforts which they often make to -procure it, when it is otherwise difficult to obtain. The beasts of prey that inhabit the central parts of the African and American continents, are known to travel immense tracts for the purpose of visiting the saltsprings that are occasionally met with, and it is said that these springs have been in some instances discovered by means of their footsteps, and by the hovering of birds over them. At the same time that we thus find animals to be led by instinct to the use of salt, we perceive that the human species are induced to employ it from its grateful effect upon the palate; for it may be remarked, that among all the singular diversities of tastes that exist among nations and individuals, there are no people, from the most barbarous to the most refined, who do not relish a certain portion of salt in their food. It may, perhaps, be thought not an unreasonable conjecture, that as salt always 4 Haller, El. Phys. xix. 3. 11; Fordyce on Digestion, p. 55. I believe that the opinion which appeared to be established by the experiments of Pringle, that although salt is powerfully antiseptic under ordinary circumstances, it promotes the decom exists in the blood and the other fluids, and must therefore be supposed to be of some essential use in the animal œconomy, there is a provision made for a regular supply of it in the constitution of our organs, and in our natural propensities and instincts. There is a numerous class of substances, which are somewhat analogous to the condiments in their effects upon the stomach, although very different in their action upon the palate, the various medicaments. These do not afford nutrition, but they many of them tend to put the stomach into a state which adapts it for the digestion of aliment, and they produce upon the system generally, or upon some of its organs, certain changes, of which we take advantage in correcting its diseased actions and restoring its powers, when perverted or weakened. The farther prosecution of this subject does not fall within the province of the physiologist, but I may observe that the operation of medicines affords us many interesting examples of the nature of the vital functions, and conversely, that a correct knowledge of these functions must very materially contribute to guide us in the selection and administration of these substances. I may remark, that condiments and medicines differ in one essential circumstance from the articles of diet; that whereas the latter are always resolved into their ultimate elements, before they contribute to nutrition, the former act in their entire state, and if decomposed, would probably cease to produce their appro position of alimentary matter, when added to them in small quantity, Appendix, p. 351, 2, is now generally supposed to be without foundation. priate effects. Some of the substances which possess the most powerful action over the system are derived from the vegetable kingdom, and are therefore composed of the same elements with our ordinary articles of food, only combined in different proportions; and even the most active mineral or metallic substances become inert when they are resolved into their elementary constituents." There is perhaps no substance whose operation on the animal œconomy is more violent than the hydrocyanic acid, yet this is entirely composed of carbon, hydrogen, and nitrogen. The acrid extracts and the narcotic alkalies that are procured from vegetables, all consist of different proportions of oxygen, hydrogen, and carbon, to which, in some cases, a quantity of nitrogen is added. The pure metals appear to exert no action upon the system, although many of their oxides and salts are so acrid. Phosphorus differs from other bodies of the same class, in being more active in its simple, than in its compound state. The three elementary bodies, chlorine, iodine, and fluorine, which agree in many of their chemical relations, resemble each other also in their powerful action on the living body, and this violence of action they retain both in their simple and in their compound form. What we commonly term poisons, are so denomi s Fordyce observes, that certain insects live entirely upon cantharides, yet their fluids are perfectly mild; On Digestion, p. 86; also that the poison of the rattle-snake is perfectly innocent when taken into the stomach; p. 119. |