86 ORGANISM AND ENVIRONMENT 



necessarily lose sight of the co-ordination which ex- 

 presses itself in the persistence or constancy of cell- 

 structure and of the internal environment. But the 

 co-ordination is plain when we look at the phenomena 

 as a whole, and becomes more and more detailed the 

 more we penetrate towards the living tissue elements. 



The phenomena of breathing have turned out to be 

 the outward expression of one side of the co-ordinated 

 activities which we lump together under the name of 

 metabolism. Our conception of breathing depends, 

 therefore, on the ideas we can form of this 

 metabolism. 



At the conclusion of this lecture let us glance at 

 what may be called physiological causation. All physi- 

 ological activities seem to be in response to external 

 or internal causes or "stimuli." Physiologists speak 

 of a "stimulus" rather than a "cause," since the word 

 "stimulus" expresses the fact that other external con- 

 ditions determine the response besides the stimulus 

 itself. The response depends, not merely on the 

 strength of the stimulus, but on the "excitability" of 

 the responding tissue. In other words the response 

 may be partially or wholly inhibited or greatly in- 

 creased by varying conditions in the environment of 

 the tissue. The character or direction of the response 

 may also depend on these conditions, or even on the 

 strength of the stimulus itself. 



As has been already shown, the respiratory centre 

 normally responds with rhythmic inspiratory and ex- 

 piratory responses to the stimulus of a very minute 



