THE RESPIRATION 291 



concluded that the nervous mechanism presiding over this 

 act is situated there. 



It must not be imagined that this centre sends fibres 

 directly to the muscles concerned in respiration. The 

 nerves passing to these come from the cells in the grey 

 matter of the spinal cord, and it is by influencing the 

 activity of these cells that the respiratory centre controls 

 the act of respiration. 



Since expiration, when forced, is a complex muscular act, 

 it is reasonable to suppose that the respiratory centre con- 

 tains two parts one presiding over inspiration, one presiding 

 over expiration. While the inspiratory centre is constantly 

 in rhythmic action, the expiratory centre is only occasionally 

 at work. 



Mode of Action of the Respiratory Centre. Both parts of 

 the respiratory centre are under the control of higher nerve 

 centres, and through these they may be thrown into action 

 at any time, or even prevented from acting for the space of 

 a minute or so. But, after the lapse of this period, the 

 respiratory mechanism proceeds to act in spite of the most 

 powerful attempts to prevent it. 



To determine its mode of action the influence of afferent 

 nerves upon the centre must be considered. 



Yagus. Since the vagus is the nerve of the respiratory 

 tract we should expect it to have an important influence on 

 the centre (Fig. 136). 



Section of one vagus causes the respiration to become 

 slower and deeper ; but, after a time, the effect wears 

 off, and the previous rate and depth of respiration is 

 regained (Fig. 137). 



Section of both vagi causes a very marked slowing and 

 deepening of the respiration, which persists for some time, 

 and passes oft slowly and incompletely. Now, if after the 

 vagi have been cut, the connection of the centre with the 

 upper brain tracts is severed, the mode of action of the 

 centre totally changes. Instead of discharging rhythmically 

 it remains for a long period at rest, then the inspiratory 

 centre discharges violently, causing a strong and prolonged 

 contraction of the muscles of inspiration. This passes off, 

 and again a period of rest of variable duration sets in, to be 



