1144 



PHYSIOLOGY 



thereby augmented to such an extent as to keep the alveolar tension of carbon 

 dioxide almost constant. If the same percentage of carbon dioxide be 

 administered to an animal after section of both vagi, the effect is deepening 

 of respiration but not quickening (Fig. 527). Each inspiratory movement 

 however is already considerable so that the margin by which increase of 

 pulmonary ventilation is possible, by increase of depth of respiration alone, 

 is not so great as in a normal animal. Moreover, since no quickening of 

 respiration takes place, the increased ventilation rapidly becomes inadequate 

 for the maintenance of the normal alveolar carbon dioxide tension. In the 

 following Table the total amounts of pulmonary ventilation, obtained on 

 administration of mixtures containing carbon dioxide to a rabbit before 

 and after section of the vagi, are compared. 



Whether we assume that the prevailing impulses travelling up the 

 vagi are purely inhibitory or are both inhibitory and augmentor, the re- 

 sultant effect, by reining in the activity of the centre, is to economise its 

 energy and the energy of the respiratory muscles. The result of the vagal 

 impulses will therefore be to increase the excitability of the respiratory 

 centre and make it more susceptible to slight changes in the carbon dioxide 

 tension of the blood, while maintaining a sufficient margin of energy to 

 meet the increased needs thrown on the respiratory mechanism by augmented 

 metabolism, such as occurs in violent muscular exercise. 



The important part played by the vagi in the regulation of normal 

 respiration is shown still more strikingly if the respiratory centre in the 

 medulla be separated from the higher parts of the brain before the section 

 of the vagi is carried out. Separation of the medulla from the higher parts 

 of the brain, as by section just behind the corpora quadrigemina, has 

 practically no influence on the respiratory rhythm. If now both vagi be 

 divided, the normal respiratory movements cease entirely, being replaced 

 by a series of inspiratory spasms, each of which lasts several seconds and 

 is followed by a pause of half to one minute's duration. These spasms are 



