INTRO D UCTOR Y. 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 katabolism 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 katabolism from secretory cells ; for it is in 
