INTRODUCTORY. 869 



up from non-living materials, furnished by the food ; and, on the other 

 hand, they are broken down into simpler substances, which pass 

 away from the tissue into the blood, and ultimately from the body 

 with the excreta, or, as in the case of secretory glands, directly into 

 secretions. The building-up process, whereby fresh molecules of bioplasm 

 are formed, has come to be spoken of as an anabolic change (anabolism, 

 assimilation), and the breaking - down process as a katabolic change 

 (katabolism, dissimilation). It is clear that these two processes will 

 produce opposite effects upon the bioplasm, the one increasing and the 

 other diminishing its bulk. But, on the other hand, it is conceivable 

 that even within the same cell there may be, at the same time, both a 

 building up or anabolic change proceeding, so that fresh molecules of 

 bioplasm are being formed, and also a breaking-down or katabolic change, 

 affecting molecules which have been formed previously, and the net 

 result to the bulk of the tissue may be nil, provided that these two 

 processes balance one another ; that is to say, the bioplasm, although 

 undergoing active metabolic changes, and furnishing products of its 

 metabolism to the secretions or to the blood, is not altered in amount 

 (autonomous equilibrium). But although both processes are occurring 

 simultaneously, they nevertheless do not exactly balance one another, 

 there will be as the net result either a gain or loss of bioplasm, i.e. the 

 bioplasm of the cell will increase or diminish in amount. If every cell 

 were entirely composed of bioplasm, this would evidently involve an 

 increase or diminution in the bulk of the cell itself. But besides the 

 actual bioplasm, all cells contain in a variable proportion products of 

 the activity of their bioplasm ; " formed material," in the sense of 

 Lionel Beale, as distinguished from " formative matter." If these 

 products remain within the cell, it may, in spite of the fact that kata- 

 bolic processes are proceeding within it more actively than anabolic 

 processes, still increase in bulk, even to a very large extent, but without 

 any corresponding increase, indeed even with an actual diminution, of 

 its bioplasm. 



Various circumstances may determine the general direction of the 

 metabolism of a cell, whether upward in the direction of increased 

 anabolism with increase of bioplasm, or downward in the direction of 

 increased katabolisni with decrease of bioplasm. One such circumstance 

 is undoubtedly the amount and nature of the pabulum supplied to the 

 cell. Another is to be found in the general physical conditions of the 

 environment, such as variations of temperature, supply of water and of 

 oxygen, and the like. And in the case of many animal cells we may 

 well suppose (and indeed the point may be said to have been determined 

 for specific instances) that impulses derived from the nervous system 

 may set up respectively, according to their nature, or the nervous channel 

 along which they are conveyed, metabolic changes in either an anabolic 

 or a katabolic direction. Thus it has been suggested by Gaskell that the 

 heart nerves act upon its muscular substance, so as to produce respect- 

 ively anabolic changes (vagus fibres, inhibitory impulses) and katabolic 

 changes (sympathetic fibres, augmentor and accelerator impulses), accom- 

 panied by diminished activity in the one case, by increased activity in 

 the other. The possibility must, however, be also borne in mind that the 

 same nerve fibres may set up both anabolic and katabolic changes, as 

 when a secretory nerve is stimulated, provoking it may be for hours a 

 discharge of products of katabolisni from secretory cells ; for it is in 



