1220 PHYSIOLOGY 



if not quite, as well as in the intact animal. The importance of 

 the vagus action for the organism is shown, however, if we put 

 an increased strain on the respiratory mechanism, as, for instance, 

 by increasing the percentage of carbon dioxide in the air breathed. 

 In the intact animal this procedure leads first to increased depth and 

 later to increased frequency of respiration, the total ventilation 

 being 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. 512). 

 Each inspiratory movement, however, is already considerable so 

 that the margin by which increase of pulmonary ventilation is possible, 



Wlti 



Mil 



WttiTOV 



FIG. 512. Effect of 10-6 per cent. C0 2 in a mixture containing 23'3-per cent. 0. 2 

 on a rabbit with both vagi divided. The gas was administered between the 

 arrows. Zero line of blood pressure is 32 mm. below bottom of tracing. 

 Compare this figure with Fig. 503, p. 1205. (F. H. SCOTT.) 



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 main- 

 tenance of the normal alveolar carbon dioxide tension. In the Table 

 on p. 1221 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 resultant effect, by reining in the activity of the centre, is to econo- 

 mise 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 



