THE CONTROL OF RESPIRATION 81 



the medulla has been derived from experiments in which elec- 

 trodes were used for either stimulation, recording, or locahsed 

 destruction of the so-called respiratory centre. Electrical stimu- 

 lation of more ventral parts produces inspiration whereas the 

 more dorsal and lateral regions are concerned with expiration. 

 Recording shows a similar distribution of active units although 

 it is by no means as clear-cut as has often been supposed. On this 

 view, which has been generally accepted until the last few years, 

 the inspiratory centre is thought to act tonically (i.e., to have a 

 continuous output of impulses) not only on the lower motor 

 neurones which produce the movements of respiration, but also 

 on the expiratory and pontine or pneumotaxic centres. The latter 

 is a centre within the pons"^ which acts back on the inspiratory 

 centre and produces periodic inhibition of the tonic outflow to 

 the inspiratory muscles. Similar inhibitory effects, terminating 

 inspiration, can be produced by the expiratory centre and by the 

 inflow from the vagus nerve which contains sensory fibres from 

 proprioceptors in the lung (probably unencapsulated endings 

 of the tracheobronchial tree). Cutting the vagi does not 

 abolish the respiratory rhythm so this input is not essential for 

 its expression. When the brain is cut in the upper brain stem, 

 following bilateral vagotomy, the animal enters a state of 

 inspiratory spasm or apneusis. This had previously been 

 thought to be due to the removal of the periodic inhibition 

 derived from the pontine or pneumotaxic centre and hence 

 the inspiratory muscles received a continuous discharge of 

 impulses. 



One of the main difficulties in accepting this theory is that the 

 so-called apneusis (meaning breath-holding) is apparently not a 

 complete inspiratory cramp but is made up of a series of very 

 small breaths. It is clear, therefore, that rhythmicity persists 

 within the medullary centres even after they are separated from 

 the pneumotaxic centre and from the vagal input. Further 

 evidence for this view came from many experiments in which it 

 was possible to restore normal breathing following these opera- 



* A projection from the under surface of the medulla below the 

 cerebellum. 



