NUTRITION. The blood which is carried by the capillaries to the several tissues of the body is the source from whence all the organs derive the materials of their growth and development; and it is found that there is direct proportion between the vascularity of any part and the activity of the nutrient operations which take place in it. Thus, in nervous tissue and muscle, in mucous membrane and in skin, a rapid decay and renova tion of tissue are constantly going on, and these are parts in which the capillaries are the most abundant; while in cartilage and bone, tendon and ligament, the disintegration of tissue is comparatively slow, and the capillaries are Much less abundant. Each elemen tary cell or particle of a tissue seems to have a sort of gland-like power not only of attracting materials from the blood, but of causing them to assume its structure, and par ticipate in its properties. Titus, from the same common source, nerves form nervous tissue, muscles muscular substance, and even morbid growths, such as cancer, have an assimilating power.
Before enteringfa:r1.her intO:the,gubject of nutrition; it isnecessarytoiunderstand 119NY it. difl9r..3 from the allied processes of development and growth. processes are the results of the plastic or assimilative 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 different conditions.
Development is the process by winch 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 development, and continues after it, is prop erly mere increase of a part by the insertion or superaddition of materials similar to those of which it already consists. In growth, properly so called, no change of form or composition occurs; parts only increase in weight, and usually in size; and if they acquire more power, it is only more power of the same kind as that which they before enjoyed.
Nutrition, on the other hand, is the process by which the various parts are maintained in the same general conditions of form, size, and composition, which they have already by development and growth attained. It is by this process that an adult person in health
maintains for a considerable number of years the same general outline of features, and nearly the same size and weight, although during all this time the several tissues of his body are undergoing perpetual decay and renovation. In many parts this removal and renewal of the particles is evident. In the glands—the kidneys (q.v.), for example—the cells of which they are mainly composed are being constantly cast off; yet each gland maintains its form and proper composition, because for every cell that is thrown off a new one is produced. In the epidermis of the skin, a similar process is perpetually going on before our eyes. In the muscles a similar change may be readily traced, for, within certain limits, an increased amount of exercise is directly followed by an increased excre tion of the ordinary products of the decomposition of the nitrogenous tissues—viz. urea, carbonic acid, and water. Again, after prolonged mental exertion, there is often a very marked increase in the amount of alkaline phosphates in the urine, which seems to show that in these cases there is an excessive oxidation of the phosphorus of the brain; and yet, in consequence of the activity of the reparative process, neither the muscles nor the brain diminish in size.
It may be regarded as an established fact in physiology that every particle of the body is formed for a certain period of existence in the ordinary conditions of active life, at the end of which period, if not previously destroyed by excessive exercise, it is absorbed or dies, and is cast off. (The hair and deciduous or milk teeth afford good illustrations of this law.) The less a part is exercised the longer its component particles,appear to live. Thus Mr. Paget found that if the general development of the tadpole be retarded by keeping it in a cold, dark place, and if hereby the funtions of the blood corpuscles be slowly and imperfectly discharged, the animal will retain its embryonic state for several weeks longer than usual, and the development, of the second set of corpuscles will be proportionally postponed, while the individual life of the corpuscles of the first set will be, by the same time, prolonged.