667 N U T E I T I N BY the term nutrition, employed in its widest sense, is understood the process, or rather the assemblage of processes, concerned in the maintenance and repair of the living body as a whole, or of its constituent parts or organs. The term has, however, usually been limited in systematic treatises on physiology to a study, mainly statistical, of the relations which exist between a living being and the medium which it inhabits, embracing a determination of the gains and losses of the organism under the different conditions to which it may be ex posed. Such a statistical study has already been pursued in the article DIETETICS, and we shall therefore in the present article, restricting our attention to the animal kingdom, consider nutrition in the more general sense above referred to, including (1) a study of the function of digestion ; (2) a sketch of the processes concerned in the absorption of matter into the blood ; (3) an account of the chemical processes which have their seat in the tissues and organs of the body ; (4) an account of the processes whereby redundant matters, or such as are the products of waste, are removed from the animal economy ; (5) a refer ence to the transformations of energy which are associated with the exchange of the matters of the body ; (6) a brief reference to the processes of growth, decay, and death. With the exception of the chemical processes concerned in respiration, which will be treated of at length under that head, this article will therefore include a short dis cussion of the chemical operations of the body generally, and particularly as they are exemplified in the case of man. General Introductory Considerations.- There is no con ception which we can form in reference to a living being, however rudimentary its structure, which is so general as the following: a living animal, so long as it manifests those attributes which characterize it as living, is the seat of continual transformations of potential into kinetic energy. Such transformations are connected with oxidation of organic matters which, primarily derived from the vegetable kingdom, have been assimilated, i.e., have been converted into the substance of the animal, and are effected through the agency of oxygen gas introduced into the body in the process of respiration. The act of living is an act of com bustion in which the animal body actually burns, and the energy at the disposal of the body, and which is employed in raising its temperature or effecting the movements which are essential to its continued life, is energy which was potential in the organic constituents burned. The main products of the combustion of the body are carbonic acid and water, besides certain other less completely oxidized substances, which are the analogues of the sooty and tarry products of combustion in a furnace. Of these products some, as carbonic acid and a part of the water formed, are removed from the body almost as soon as they are formed, while others are thrown off at intervals. It follows from what has been stated that the act of living necessarily implies not only transformation of energy but actual waste of the matter of the body, and that if an animal is continuously to manifest the pheno mena of life it must be supplied with oxidizable organic matter to take the place of that which has been oxidized. Accordingly, animal life is impossible unless the creature, besides receiving continual supplies of oxygen gas, receives at intervals supplies of food. The food of an animal con sists (1) of oxidizable organic matters which, although they may have been derived in part or wholly from some other animal body, have primarily been built up through the instrumentality of vegetable organisms, these organic matters belong to a few well-defined groups ; (2) of mineral matters, including large quantities of water, which form an important part of the substance of the body, and the presence in and passage of which through the organism is essential to the physical processes which have their seat in it. In brief, the animal body is the seat of processes of dis integration associated with the manifestation of kinetic energy, and of processes of integration in which oxidizable matters take the place of the oxidized constituents. If life is to continue, in a sense, indefinitely it is essential that the processes of integration and disintegration should balance, i.e., that the receipts of the body in assimilable oxidizable matter should balance the expenditure of the body both in matter and energy. There are, it will be observed, very close analogies between an animal and such a mechanism as a steam-engine, the energy at the disposal of both being primarily derived from the oxida tion of combustible matters. Some of the most salient points of difference must, however, not be lost sight of. (1) The waste of the essential parts of such a machine as a steam-engine is insignificant, and bears no definite rela tion to the work done. The kinetic energy of the machine is primarily due to oxidation processes taking place in the furnace, and in no respect to changes in the substance of the machine. The animal, on the other hand, wastes con tinuously in all its parts and organs, and much of its energy is derived immediately from material which has become part and parcel of the various mechanisms. (2) Any sub stance capable of being rapidly oxidized (burned), and thus of generating heat, may be used as fuel for a steam- engine, provided its combustion admits of being conducted with safety in its furnace, whilst the substances which can form the food of animals belong to a limited number of groups, which include but a comparatively small number of bodies. The constituents of food have not only to supply energy to the body, but they must further be cap able of prior conversion into the very substance of the animal body, into its very " flesh and blood." Moreover, the constituents of food must be free from all traces of the peculiar substances which we term "poisons," and which by their presence have the power of impairing and arresting the action of various organs of the body. I. FUNCTION OF DIGESTION. Hunger and Thirst. These terms are used to express peculiar sensations which are produced by and give ex pression to general wants of the system, satisfied respect ively by the ingestion of organic solids containing sub stances capable of acting as food, and by water or liquids and solids containing water. Hunger is a peculiarly indefinite sensation of craving or want which is referred to the stomach, but with which is often combined, always indeed in its most pronounced stages, a general feeling of weakness or faintness. The earliest stages are unattended with suffering, and, leading the animal to wish and seek for food, are characterized as "appetite for food." Hunger is normally appeased by the introduction of solid or semi-solid nutriment into the stomach, and it is probable that the almost immediate alleviation of the sensation under these circumstances is in part due to a local influence, perhaps connected with a free secretion of gastric juice. Essentially, however, the sensation of hunger is a mere local expression of a general want, and this local expression ceases when the want is satisfied, even though only liquid and no solid food is introduced into the stomach, or even though no food be introduced into the stomach, the needs of the economy being satisfied by the introduction of food through other
