Page:The American Cyclopædia (1879) Volume XII.djvu/558

 544 NUTRITION prevent it by not making room for assimilated tissue. Therefore, aside from stimulating the circulation of the blood, the influence of exer- cise upon the nutritive functions is of great importance as an aid in eliminating effete mat- ter. The formation of abnormal growths is a variation of the nutritive process which will be found treated under the heads CANCER, EXOSTOSIS, TUMOR, &c. In cold-blooded ani- mals 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 win- ter enter into a condition of hibernation, during which time the fat and other tissues take the place of food as supporters of organic life du- ring the temporary suspension of ingesta. (See HIBERNATION.) The discussion of the ques- tion 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 develop- ment of living organisms, inasmuch as it in- volves 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 ex- plainable by physical theories ; as it is not diffi- cult 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 cer- tain limits. The interdependence of the vari- ous 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 temper- ature between certain limits, is also a necessary condition of nutrition ; digestion cannot be ac- complished 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 dimin- ished 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 metamor- phosis of tissue, whereby latent is converted into sensible heat. That it nevertheless reacts, to interfere with assimilation, cannot be doubt- ed ; 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 pro- cesses of fermentation and for the various chem- ical transformations. Cold produces numb- ness, 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, espe- cially 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 with- in the tissues themselves. As the consump- tion of oxygen forms a part of the processes by which nutritive material is prepared for assimilation, it really forms a part of the in- come 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 ali- mentary principles which are capable of im- mediately generating force, but also with other substances, such as water and various saline bodies, and is greatly dependent upon the di- gestibility 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 vegeta- tive functions, their effect being to raise prox- imate 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 forma- tive processes of nutrition may therefore be considered as supplementary to those forma- tive 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 re- duced by animal metamorphoses to a lower' degree, with the evolution of force. Thus the conversion of sugar into fat by animal diges- tion and assimilation, by which CeHisOe (glu- cose or starch sugar) is converted into CieH^Oa (oleic acid), OsHsOs (glycerine), and other con- stituents of fat, is, as will be seen from the for- mulas, a deoxidizing process, by which a prox-