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. 



