274 



ESSENTIALS OF PHYSIOLOGY4 



respiratory movements, they are not essential, and respiration continues 

 after they are prevented from reaching the respiratory centre by 

 division of the vagi. Their principal function seems to be that of 

 rendering the centre more sensitive to the normal stimulus of carbonic 

 acid, and of controlling the respiratory discharge from the centre to 

 the respiratory muscles. In their absence, the respiratory centre is not 

 stimulated until the tension of carbonic acid in the blood rises higher 

 than in the normal animal, although the resulting respiration, when it 

 does occur, is exceedingly forcible ; moreover, the normal adjustment of 

 the respiratory movements in response to any considerable increase in the 



FIG. 109. Respiratory movements in rabbit with vagi divided. Between the arrows, 

 the inspired air contained 10 per cent. C0 2 . Note that the rate of respiration 

 is not increased. (Scott. ) 



tension of carbonic acid in alveolar air and blood is no longer efficiently 

 carried out. 



During muscular exercise, for example, a normal animal breathes 

 not only more deeply, but also more frequently, and is thus able to 

 expel from the lungs all the additional carbonic acid reaching them 

 from the blood ; after section of the vagi the rate of respiration remains 

 unaltered, the ventilation of the lungs is inadequate, and the percentage 

 of carbonic acid in the alveolar air rises. The same result is seen 

 when an animal, with its vagi divided, breathes air containing an excess 

 of carbonic acid (fig. 109), the ventilation of the lungs being much less 

 than in the normal animal, as is seen in the following table : 



VENTILATION OF THE LUNGS. 



