In these ways, the blood and lymph are continually supplied with proteid, fat, and carbohydrate from the ingested food, and as these fluids surround and permeate the organized elements of the tissues, the latter are enabled to gain what they need to maintain their nutritive balance. Living matter is essentially unstable; it is the seat of chemical changes of various kinds, anabolic or constructive, and katabolic or destructive. The more comprehensive term “metabolic” is applied to all of these changes that take place in living matter. In anabolism, the dead, inert proteids, fats, and carbohydrates are more or less assimilated and made a part of the living matter of the tissue cells, while at the same time a certain amount of the food material, probably the larger amount, is simply stored as such, or left to circulate in the blood and lymph, without being raised to the higher level of living protoplasm. In katabolism, this accumulated material, and in some degree the living substance itself, is broken down or disintegrated with liberation of the stored-up energy, which manifests itself in the form of heat and mechanical work. At times, the anabolic processes predominate and there is a relatively large accumulation of stored-up materials; while at other times, katabolism, with its attendant chemical decompositions, predominates, and the body loses correspondingly. The point to be emphasized here is that the living body, with its multitude of living cells, is the seat of incessant change. Construction and destruction are continually going forward side by side; sometimes the one and sometimes the other predominating, according to existing conditions. The living protoplasm with its attendant storage material is, under ordinary conditions, constantly being made good from the assimilated food, a part of which is raised to the dignity of living matter and becomes an integral part of the living cells, while the larger portion is simply stored for future uses, or circulates in the blood and lymph which bathe them. Doubtless, this storage or circulating material is the main source of the energy which constantly flows from the cells in the form of heat and of work, as a result of the disruptive changes that constitute katabolism.