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 



