ORGANIC REGULATION 91 
structural change. Similar structural change is known 
to result from many other slight alterations in the 
composition of the blood; and so far as the evidence 
goes, it points to the conclusion that the specific struc- 
ture of every part of the body depends upon the spe- 
cific composition of the blood, as well as on the influ- 
ence of the adjacent tissues or external environment. 
The regulation by the tissues and organs of the inter- 
nal environment is thus only their regulation of their 
own structure and activity. 
A living organism has, in truth, but little resemblance 
to an ordinary machine. The individual parts of the 
latter are stable, within very wide limits of immediate 
environment, and in no way dependent on whether 
the machine is in action or at rest. This stability does 
not exist in the living organism. We find, it is true, 
that the living organism may react in a constant man- 
ner to a given change, just as a machine might do; but 
on investigation this turns out to be because the inter- 
nal environment is at the time constant or “normal.” 
Were it otherwise not even the superficial resemblance 
would hold. As we have seen, for example, in the 
case of the respiratory centre, this reasoning applies 
to nervous reactions just as much as to other physi- 
ological reactions. 
It seems clear, therefore, that we cannot base our 
explanation of the constancy of the internal environ- 
ment on the structure of the organs which regulate 
it, since closer examination shows that the “structure” 
of these organs is itself dependent on the constancy 
of the internal environment. We are only reasoning 
