Nutrition, the growth and reparation of living organisms, animal and vegetable. Animal nutrition in its most extended sense includes the various complex processes of digestion, chylification, sanguification, circulation, respiration, assimilation, secretion, and excretion. In a more restricted sense it is the conversion of nutritive material into the various tissues of the body. The first important process of nutrition is digestion (see Digestion); the next is the conversion of the digested material into blood, or the process of sanguification; the third is the formation of bodily tissue from the constituents of the blood (assimilation), which is done by virtue of the power of selective appropriation by the tissues themselves. The materials appropriated by the organism may be divided into two kinds, the nitrogenous or pro-teinaceous, and the non-nitrogenous or hydro-carbonaceous. This branch of the subject will be found treated under the heads Aliment, Animal Heat, and Dietetics. The action of the nervous system has much to do with the functions of nutrition, principally because of the influence the nerves have upon the circulation of the blood.

That nervous condition which causes an increased circulation of the blood in a part will, if continued, cause its larger development, instances of which are seen in the arms of blacksmiths and the legs of dancers. Therefore exercise or training becomes an important element in influencing the nutrition and development of the whole or parts of the body. Disassimilation or the disintegration of structure, the initiative process of excretion, must always accompany a continuance of nutrition, because the detention of excrementi-tious matter would not only poison the fluids, and in this way prevent assimilation, but would prevent it by not making room for assimilated tissue. Therefore, aside from stimulating the circulation of the blood, the influence of exercise upon the nutritive functions is of great importance as an aid in eliminating effete matter. The formation of abnormal growths is a variation of the nutritive process which will be found treated under the heads Cancer, Exostosis, Tumor, etc.

In cold-blooded animals nutrition may be greatly retarded and some of the functions suspended by a greatly diminished temperature; and this to a certain extent is true of some warm-blooded animals, as the bear and the woodchuck, which in winter enter into a condition of hibernation, during which time the fat and other tissues take the place of food as supporters of organic life during the temporary suspension of ingesta. (See Hibernation.) The discussion of the question of increased nutrition during infancy and youth, and of the balance between nutrition and waste during the prime of life, is full of interest, but does not properly find place in this article. It is attended by a consideration of questions of a philosophical nature which greatly concern the reception or rejection of modern theories of the generation and development of living organisms, inasmuch as it involves the explanation of limitation of growth by purely molecular forces. The assimilation by each organ or tissue of material of the same nature with itself is a process more easily explainable by physical theories; as it is not difficult to comprehend that histological structures may exert an influence on contiguous formative matter of similar composition, whereby it may be caused to assume a similar development.

The great purpose or end of nutrition is to evolve certain vital phenomena which depend upon a variety of molecular changes requiring the maintenance of a temperature within certain limits. The interdependence of the various vital processes which are carried on in the animal system renders it difficult to form an estimate as to which are of primary and which of secondary importance. Thus, among the forces either directly or indirectly evolved by nutrition is heat. But heat, that is, a temperature between certain limits, is also a necessary condition of nutrition; digestion cannot be accomplished outside of these limits; and the same may be said of sanguification. In fevers, which as a rule are attended with increased bodily temperature, there is also greatly diminished assimilation. It must not be concluded, however, that the increased temperature is a primary cause of the diminished assimilation; on the contrary, it is to be considered as a result of the altered assimilation and metamorphosis of tissue, whereby latent is converted into sensible heat. That it nevertheless reacts, to interfere with assimilation, cannot be doubted; but the extent of its influence is difficult of estimation.

Innervation also depends as well upon the maintenance of a temperature between certain limits as upon assimilation and metamorphosis of tissue; and on the other hand, these processes are dependent to a great extent upon innervation. A certain degree of heat is as necessary for the performance of the functions of nutrition as it is for the processes of fermentation and for the various chemical transformations. Cold produces numbness, and advantage is taken of this action by the employment of refrigeration in surgical operations. The functions of sanguification and assimilation may, however, be considered as the two most important to nutrition, especially if we restrict the term as denoting simply a formative process. It may be remarked that sanguification is accompanied by a passage of matter, when becoming plastic, into the blood vessels, while assimilation is accompanied by the passage of the plastic portions of the blood out of the blood vessels through the walls of the capillaries, and virtually takes place outside of the circulatory apparatus and within the tissues themselves.

As the consumption of oxygen forms a part of the processes by which nutritive material is prepared for assimilation, it really forms a part of the income of the system, and is therefore nutritive material; but it stands so distinctly apart from the other materials, that these latter, composed of animal and vegetable substances, are by common consent classed as articles of food, or alimentary principles. They have the property of being digested, that is, of being dissolved by the gastric juice and other digestive fluids, and rendered capable of absorption by the lacteal and capillary vessels, and are all capable of oxidation. Nutrition demands that the system shall be supplied not only with oxidizable alimentary principles which are capable of immediately generating force, but also with other substances, such as water and various saline bodies, and is greatly dependent upon the digestibility of food, not so much as regards time of digestibility as degree. The functions of digestion, sanguification", and assimilation are regarded as having the nature of the assimilative processes in plants, and are often called vegetative functions, their effect being to raise proximate organic principles to a condition which will permit them in undergoing oxidation, or any mode of metamorphosis, to develop some form of vital or physical force.

The formative processes of nutrition may therefore be considered as supplementary to those formative processes which take place in the vegetable kingdom, by which proximate elements, under the influence of light and heat, are formed from inorganic nature and raised to a higher degree of potentiality, which potentiality is again reduced by animal metamorphoses to a lower degree, with the evolution of force. Thus the conversion of sugar into fat by animal digestion and assimilation, by which C6H12O6 (glucose or starch sugar) is converted into CisHs-iOa (oleic acid), C3H803 (glycerine), and other constituents of fat, is, as will be seen from the formulas, a deoxidizing process, by which a proximate principle is raised to a higher potential. This raising of potential by organization, whether in the plant or animal, may be regarded as a conversion of force into matter, while the animal metamorphoses of tissue may be regarded as a conversion of matter into force, or more strictly speaking the evolution of force by matter. The precise point at which the vital transformations begin to generate force would be difficult, perhaps impossible, to determine, and probably varies under different circumstances.

Thus the absorption of oxygen by the blood globules should be regarded as a force-generating process; while in the nervous system the evolution of force is to be regarded as commencing with the metamorphosis or oxidation of nerve matter. According to the experiments of Bischof and Voigt, it is concluded that all the nitrogenous material which is digested and taken into the circulation is assimilated into flesh (not limiting the term to muscular fibre, but including all proteine tissues) before it undergoes metamorphosis into urea. The first series of experiments made were with an exclusively meat diet, and the first and most marked effect was an increase in the production of urea. If, at the time of commencing the experiment, the dog was ill fed and losing in weight, the feeding of a small quantity of lean meat caused such an increase in the waste (metamorphosis) that nothing was stored up and the animal continued to lose weight. An increase in the quantity of meat caused an increase in the metamorphosis and a continued loss of weight; but it was found that a continued succession of equal increments was not accompanied by equal increments of metamorphosis, but that the latter diminished, so that at length a point was reached when the quantity of ingesta balanced the amount of metamorphosis.

This condition was established only when the amount of meat eaten by the dog was equal to 1/20 or 1/25 of his weight. An increase beyond this caused an increase in weight; but in a short time another equilibrium was reached, and a succession of increases of weight followed by states of equilibrium occurred, each state of equilibrium occupying a higher level or potential, until at last a point was reached when the animal refused to take the required quantity of food. Then followed a loss of weight and a reduction of equilibrium to a lower level. The cause of the successive diminution in the increments of metamorphosis compared to the increments of food is found in the nearly equable quantity of oxygen present in the blood. In the next series of experiments fat and lean meat were given together, and then fat alone. The adddition of fat to meat produced two different effects. The fat did not prevent the increase of metamorphosis which took place when lean meat alone was used, but rather increased it; at the same time, however, on account of its greater readiness to combine with oxygen, it protected the flesh from the action of this agent.

It was found that only one third as much lean meat was required to maintain equilibrium when fat was added as when not., Sugar and starch were found to have a similar effect, but in a rather greater degree. These experiments accord with the results of Mr. Banting. (See Abstinence, Corpulence, and Banting.) Ranke found that in man, who is omnivorous, an equilibrium could not be maintained on lean meat alone, and that a loss of weight occurred even when eating the greatest quantity possible; but by the addition of fat or starch a state of equilibrium or increase was easily attainable. - The production of fatty matter by insects in excess of the fat contained in their food was established long ago by the experiments of Huber on bees, and confirmed by Dumas and Milne-Edwards. The experiments also of Boussingault upon pigs, whose digestive organs resemble those of man, establish the fact that fat is developed in their bodies. His experiments also indicate that fat may be produced from exclusively nitrogenous food, although more readily formed from that which is simply hydrocarbonaceous. - For a further consideration of the subject, see "Physiology of Man," by Austin Flint, jr., M. D. (1866-'75); "A Treatise on Human Physiology," by John C. Dalton, M. D. (latest ed., 1870); and " Principles of Human Physiology," by W. B. Carpenter (latest ed., 1874). The subject of vegetable nutrition is treated in the article Plant.