204 C. M. CHILD 
We have seen that structure-formation of some sort, 7. e., the 
accumulation of relatively inactive substances in or about the cell 
is a characteristic feature of metabolism. The close relation be- 
tween the syntheses and the oxidation processes has been pointed 
out repeatedly by Loeb as well as by others. The structural 
substances, when once formed, play only a relatively small part in 
further metabolism, provided other more active substances, 7. é., 
nutritive materials, are at hand, and provided the general char- 
acter of metabolism is not changed. Any condition, e. g., the 
‘functional stimulus’ which leads to increased metabolic activity 
of the particular kind which constitutes what we call the special 
function of the cell or part leads, when nutritive material is pres- 
ent, to increased accumulation of the inactive substances and 
hypertrophy is the result. On the other hand, in the absence of the 
functional stimulus, or when its frequency or intensity is decreased, 
the use of nutritive material and the accumulation of structural 
substance do not occur or are less rapid, and the result is that 
below a certain level of functional activity the gradual breaking 
down of the accumulated substance, which is not immediately 
connected with the special functional activity of the part, exceeds 
the constructive processes and decrease in size and atrophy occur. 
The constructive processes continue only, or very largely, in 
connection with the functional stimulus and, for the addition of 
new structure nutritive material must be taken in from without, 
but this functional activity does not under these conditions, 
increase proportionally the rate of reéntrance of the structural 
substances into metabolism; in fact, if other more active sub- 
stances are present in sufficient quantity, the structural substances 
may be spared to a large extent. 
Hypertrophy and atrophy are then the result of two different 
kinds of processes, the one connected with the specialized function 
of the part in its relation to other parts, the other to a considerable 
degree independent of this except in starving animals. In its ‘fune- 
tional activity’ the part builds structure, but does not destroy it to 
so great an extent. The destructive process is largely independent 
of function and goes on more or less continuously. Whether hy- 
pertrophy or atrophy shall occur in a given case depends merely 
