S70 METABOLISM. 
such cases necessary to assume a continuous process of anabolism going 
on at the same time within the same cells. 
Upon evidence founded mainly, but not exclusively, upon the investigation 
of certain electrical and visual phenomena, Hering has concluded that in all 
cases where either katabolic or anabolic changes are proceeding in any portions 
of bioplasm, they tend to render the bioplasm more and more resistant to the 
effects of the excitation which is producing the change (reaction) ; that in any 
given cell the longer or more strongly metabolic changes of the one character 
have been proceeding, the greater will be the tendency towards metabolic changes 
of the opposite character, so that even if, as may happen, in consequence of the 
action of an external stimulus (A), anabolic changes are proceeding at tirst more 
rapidly than katabolic, so that the balance is in favour of the building up 
or assimilation processes, the reaction which is thereby provoked will, after a 
time, by increasing the katabolism of the cell, tend again to produce a condition 
of balance. Only in this case the balance will be struck Avith the general 
bioplasm of the cell in a condition above par, as compared with that from which 
it was assumed to start (A — allonomous equilibrium). And, mutatis mutandis, 
increased katabolic processes due to external stimuli are (D) assumed to produce 
by reaction an increase of anabolism in adjacent portions of bioplasm, which 
increase becomes eventually sufficient to balance the increased katabolism 
induced by the stimulus, so that again the metabolism of the whole cell strikes 
a balance as it were, but now in a condition below par, as compared with the 
normal (D — allonomous equilibrium). Upon the cessation of the stimulus in 
either case, the tendency, say, to increased anabolism being removed with the 
stimulus, the opposite condition of increased katabolism, which was provoked 
by the increased anabolism, will for a time prevail, and there will be a falling 
off of the general assimilation of the cell, until what may be considered the 
normal condition is again established, the two processes again exactly balancing 
one another. And the same, mutatis mutandis, for the removal of a stimulus 
which was producing a condition of increased anabolism. There is thus 
assumed to be a sort of internal self-adjustment of metabolism in bioplasm. 
It is a part of the theory of Hering that the anabolic and katabolic changes 
in the bioplasm are the direct or indirect cause of many, if not of all, 
physiological phenomena exhibited by living tissue, and that the prevalence 
of one kind of change in any portion of bioplasm will tend to start a 
change of the opposite kind in adjacent portions. But this is a subject 
which we need not here specially concern ourselves with, since the most 
important application of it to the explanation of physiological phenomena 
concerns the effects produced by the stimulation of the retina by light, and 
will be discussed in the article dealing with this question. 
In connection with this subject, one other point must be borne in mind, 
namely, the possibility, indeed probability, that many metabolic changes in the 
body are not necessarily associated with the building up or breaking down of 
bioplasm, but are effected outside the actual molecules of which the bioplasm 
is composed, although under the iniluence of the activity of the bioplasm. 
Such changes as these may be distinguished from the metabolic changes of the 
bioplasm itself by the name of " contact changes," and they also involve both 
the building up of complex materials and the subsequent breaking down of 
such materials into simpler products associated frequently with oxidation. 
Such contact changes are analogous to those which are produced by organised 
ferments, such as yeast, outside the actual organism, although directly by its 
activity, and they must be sharply differentiated from the changes which the 
bioplasm itself is at the same time undergoing. This distinction will be 
referred to again in a subsequent section. 
The understanding of the metabolic processes presiqjposes an acquaintance 
with the composition of the foodstuffs and of the bodystuffs, both of which 
