NUTRITION (from Lat. aufrire, to nour ish). The process by which living organisms appropriate, modify, and utilize the materials needful for their existence, growth, and develop ment. The ultimate appropriation of food takes place in the individual cell, which seems to have a gland-like power not only of attracting mate rials from the blood, but of causing them to assume its structure and participate in its prop erties. A necessary complement to the process of assimilation is that of excretion, which con sists in the discarding of effete matter—the prod ucts of its own vital activity—by the cell. III modern terminology the assimilative or building up process is called anabolism; the disassimila live or breaking down process is called katab olism, and the sum of the two, metabolism. The blood is the medium through which nutritive materials are brought to the cells and excreted products are carried off. It is borne by the capillaries to the several tissues of the body and is the source from which they derive the mate rials for their growth and development; and there is a direct relation bet wem the vascularity of any part and the activity of the nutritive operations which take place in it. Thus in muscle, skin, and mucous membrane, and in nerve tissue, rapid decay and renovation are constantly going on, and in these tissues the capillaries are most abundant; while in cartilage and bone, tendon and ligament, disintegration is comparatively slow, and the capillaries fewer.
All the processes of development and growth are the results of the plastic or assimila tive force by which living bodies are able to form themselves from dissimilar materials (as when an animal subsists on vegetables, or when a plant grows by appropriating the elements of water, carbonic acid, and ammonia) ; but they are the results of this force acting under differ ent conditions.
Development is the process by which each tissue or organ of a living body is first formed, or by which one, being already incompletely formed, is so changed in shape and composition as to be fitted for a function of a higher kind, or finally is advanced to the state in which it.
exists in the most perfect condition of the species.
Growth, which commonly concurs with de velopment, and continues after it, is properly mere increase of a part by the insertion or super addition of materials similar to those of which it already consists.
Nutrition, on the other hand, is the process by which the various parts are maintained in the same ;_Roleral conditions of form, size, and eomposit ion.
In the elementary forms of animal and vege table life, represented on the one hand by the annebit and on the other by bacteria, the process of nutrition is a comparatively simple one. They are surrounded by a material which they can use as food, and each individual cell, being fitted to digest and absorb, appropriates what it needs and rejects what it does not require. In the higher animals, however, careful selection and a high degree of preparation and modification of the food is necessary; and these processes are carried on in the digestive canal and by the secretions of the glands (9.v.) which open into it. The phe nomena attending the elaboration of food and its preparation for assimilation by the tissues are described under DIGESTION, I iRGA NS OF.
In order that nutrition may be carried on in a healthy manner, certain conditions must be present. The blood must be normal in composi tion and amount, and circulate with suitable rapidity; there must exist a. certain nervous stimulation and control; and the part to be nourished must be aide to appropriate the ma terials brought to it by the blood. The depend ence of nutrition upon the first of these condi tions is shown in anemia, in which disease the contents and carrying capacity of the blood are lowered, and nutrition correspondingly depressed. The influence of the Dell:MIs system is demon strated by the atrophy or even death of a part which follows the destruction or cutting off of its nerve supply. This often happens in certain diseases of the spinal cord.