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 
