METABOLISM (Greek perafloA4 a change), the whole series of physico-chemical pro cesses connected with the manufacture of protoplasm and with nutrition and growth. Under the term is included all those changes by which various energy transformations are made possible in the human body. It has been divided into constructive metabolism (anabo lism), and destructive metabolism (katabolism). The former comprises the processes by which the substances taken as food are converted into protoplasm, while katabolism is the means by which the protoplasm is broken down into simpler products, such as the excretory or waste matters. Metabolism thus comprises the ele mentary vital phenomena of plant and animal life. These are the occurrences associated with changes of substance, of form and of energy. All organisms undergo continual changes of their substance or protoplasm, etc., alterations of their form and transformations of the energy which they get from the environment. The human body is essentially an energy trans former. Thus as regards food, since all living matter is continually undergoing decomposition, it must take in substances that contain all the chemical elements of which it is constructed, hut the food differs with every form of cell. Plants live chiefly on inorganic substances and animals on organic, that is, material already prepared; no animal being able to live on simple inorganic compounds. Among animals there is a remarkable adaptation to a single kind of foot For example the larva of the fur-moth lives exclusively upon the hairs of fur, which consist of pure keratin ; this substance, which is closely allied to proteid, is therefore capable of furnishing all the elements necessary for the formation of the protoplasm of this caterpillar. Dogs when worked hard have been found to be able to live on pure proteid food. On the other hand, according to Verworn, it is im possible for an animal to live solely on carbo hydrates or fats, or even on the two together, since there is no nitrogen in these substances.
Vital motion or metabolism is a complex sharply, characterizing living organisms, and the taking in of food and the excretion of waste products give to the organism the material with which to regenerate itself and to grow. Metab olism is increased by heat, the consumption of oxygen being increased in cold-blooded animals, but warm-blooded animals undergo a decrease under heat, Thus man in winter has a much more active metabolism than in summer, since he consumes more food at low temperatures.
It is claimed by Verworn that every species of animal possesses a specific metabolism, and that under certain conditions products of metab olism may stimulate an animal to increased growth, while under other circumstances they may retard growth. Thus the usually deadly uric acid in moderate amounts exerts a favor able influence on the size of larvae of sea urchins. Illustrations are afforded by experi ments in raising snails and water-fleas (Daph nia) in small volumes of water; besides the diminished volume, the water fouled by their excretions may not only dwarf but gradually kill them, or produce changes of form. Metab olism is more active in the young than in the adult. Between 9 and 14 a boy requires as much food as a full-grown man, and during the next five years more than a man. The same is not true of girls who increase the amount of metabolism up to the 11th year.
Consult Verworn, 'General Physiology' (New York 1899) ; Vernon, 'Variation in Ani mals and Plants' (New York 1903) ; Starling, 'Human Physiology' (Philadelphia 1915) ; Bayliss, 'Principles of General Physiology' (New York 1915).