26 ORGANISM AND ENVIRONMENT 
breaths cannot follow quickly enough the requirements 
which are easily met by an animal with intact vagi. 
In the regulation of breathing we have thus a strik- 
ing instance of the co-ordination between the actions 
of two different nervous'stimuli. The influence of the 
peripheral stimuli acting through the vagus nerves is 
dependent upon the action of the central stimuli, and 
vice versa. This interdependence is characteristic of © 
the effects of nervous stimuli and indeed of all phys- 
iological stimuli. As an outcome of the interdepend- 
ence in the present case, the breathing organs work 
as a whole, the discharges from the respiratory centre 
being correlated with the actual movements of the 
lungs. 
Even after the vagi and nearly all other nervous 
connections to the respiratory centre are severed, alter- 
nate inspiratory and expiratory discharges from the 
centre continue in their proper order. The inspiratory 
discharge seems during its continuance to inhibit ex- 
piratory discharge, and vice versa. Here, also, we see 
the co-ordination which is inherent in all physiological 
activity, and which manifests itself even in the be- 
haviour of an isolated heart or strip of muscle, but far 
more strikingly in the case of the nervous system, even 
after great mutilation, or in the case of the chemical 
activities of any living part of the body. 
