174 Cc. M. CHILD 
and highly labile molecule, and on the other, of the breaking down 
of this molecule in functional activity. According to this view the 
colloid structure built up is used in function and must be continu- 
ally replaced. | 
During recent years, however, many facts have been discovered 
which seem to make necessary some modification of this view of 
the relation between metabolism and colloid structure. In the 
first place, most proteids, and indeed most organic colloids, are 
relatively inactive chemically. The common interpretation of this 
fact has been that death involved, or perhaps consisted in a change 
from lability to relative stability of the proteid molecule. But the 
‘recent work of Fischer and others upon proteids makes it highly 
probable that the proteid molecule, although very large, is not so 
complex as has been supposed, but may be polymeric in high 
degree. Thus the assumption of extreme lability of this molecule 
in the living organism becomes even more difficult than before. 
Work along other lines has demonstrated that the nitrogen me- 
tabolism is only a fraction of the total metabolism of the organism 
and that it does not necessarily increase in proportion to functional 
activity. Moreover, the nitrogen requirement for maintenance in 
animals is apparently much smaller than had been supposed. All 
of these facts seem to indicate that in the living organism, as 7n 
vitro, the proteids, or many of them, are relatively inactive chemi- 
cally; that after they are formed, they are excluded to a large 
extent from metabolism simply because of their relative inactivity. 
If these conclusions be correct, the accumulation of proteids 
in the organism is a process not very different from the deposition 
of other forms of inactive substance in or about the cell, e. g., 
chitin, insoluble salts, ete. Indeed it is highly probable that the 
accumulation of most or all structural substances in the organism 
is due to the fact that they are relatively inactive under the exist- 
ing conditions. After they have arisen in metabolism they per- 
sist or disappear much more slowly than other substances, stmply 
because under the existing conditions they do not enter chemical 
reactions as readily as other substances. 
But the proteids and other substances deposited in the cell are 
not absolutely inactive and undoubtedly do enter metabolism 
