5 o8 THE POPULAR SCIENCE MONTHLY. 



to distinguish, between them as chemical, physical, or more strictly 

 biological, are conveniently expressed by the general term metab- 

 olism. 



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 forth- 

 with 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 proto- 

 plasm are not diminished as rapidly as the katabolic transforma- 

 tions 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 Amozba, are still carried on through the 

 agency of protoplasm, but it is distributed to various organs, each 

 of which has a special function. 



* Trans. A. A. A. S., 1890, p. 283. 



