Page:Popular Science Monthly Volume 43.djvu/524

508 to distinguish between them as chemical, physical, or more strictly biological, are conveniently expressed by the general term metabolism.

Dr. M. Foster says: "We may picture to ourselves this total change which we denote by the term 'metabolism' as consisting, on the one hand, of a downward series of changes (katabolic changes), a stair of many steps, in which more complex bodies are broken down into simpler and simpler waste bodies, and, on the other hand, of an upward series of changes (anabolic changes), also a stair of many steps, by which the dead food, of varying simplicity or complexity, is, with the further assumption of energy, built up into more and more complex bodies. The summit of this double stair we call 'protoplasm.' Whether we have the right to speak of it as a single body in the chemical sense of that word, or as a mixture in some way of several bodies; whether we should regard it as the very summit of the double stair, or as embracing as well the topmost steps on either side, we can not at present tell. Even if there be a single substance forming the summit, its existence is absolutely temporary: at one instant it is made, at the next it is unmade. Matter which is passing through the phase of life rolls up the ascending steps to the top, and forthwith rolls down on the other side."

The greater activity of the nutritive processes in young and growing animals, with a gradual decline to maturity and old age, are matters of common observation. Dr. Minot has, however, shown that "with the increasing development of the organism and its advance in age we find an increase in the amount of protoplasm. This seems to indicate that katabolism is relatively more active in young organisms, and that they use protoplasm in tissue-building as fast as it is formed. In old age, on the other hand, the anabolic processes resulting in the formation of protoplasm are not diminished as rapidly as the katabolic transformations of protoplasm into new tissues, to replace the waste arising from the wear and tear of the system, and a general decline of the bodily powers follows.

As we pass from the simpler to the higher forms of life we find a gradual transition from the comparatively homogeneous protoplasm of the lowest, to the highly differentiated protoplasm of the highest forms which provide for a division of labor in the physiological activities of the different organs. In the highest organisms the functions of prehension, digestion, assimilation, respiration, etc., as in the Amœba, are still carried on through the agency of protoplasm, but it is distributed to various organs, each of which has a special function.