312 EXTERNAL RESPIRATION 



This work is almost entirely performed by the diaphragm. 

 The other muscles concerned, whether synergic or antagonistic, 

 seem to play an almost passive part. This may be inferred from 

 the fact that although they are skeletal in structure yet they 

 undergo constant slow contraction without showing fatigue. 

 When the respirations are forced the subsidiary musculature has 

 to perform work and the CO 2 output increases. The effort 

 sooner or later brings on fatigue. Forced respirations are carried 

 out uneconomically, i.e. at a relatively higher cost per litre than 

 ordinary quiet ventilation. 



Regulation of Respiratory Rate. The activity of the respiratory 

 centre, which lies in the medulla near the root of the vagus, is 

 normally governed by the tension of the CO 2 in the blood going 

 to it. The rate of breathing is increased by any increase in 

 the CO 2 tension ; and, conversely, diminution of the CO 2 tension 

 leads to a decreased respiratory rhythm. 



The CO 2 tension of the blood and the partial pressure of the 

 CO 2 in the alveolar air are, as we have explained (Chap. XXIII.), 

 always in dynamic equilibrium, and, therefore, any change in the 

 one will lead to corresponding changes in the other. It has been 

 found that an increase of 0'2 per cent, in the CO 2 of the alveolar 

 air, i.e. a rise of tension of from 40 to 41' 6 mm. Hg, is sufficient 

 to double the rate of respirations. The increased ventilation 

 leads to a " washing out " of CO 2 from the blood and from the 

 lung, thus rapidly restoring a normal condition. The power 

 of adjustment is so extraordinarily effective that under wide 

 variations of metabolic and atmospheric conditions, the tension 

 of CO 2 in alveolar air is maintained at an almost constant level 

 of about 40 mm. Hg (see p. 334). 



Regulation oJ Depth. Impulses are constantly passing from 

 the lungs, through the vagi, to the respiratory centre and, by a 

 reflex act, inspiration is checked when a certain tension is set 

 up in the lung substance, i.e. the respiratory mechanism carries 

 out the inspiratory phase of its function till this stretch-reflex 

 inhibits it. In cases where the vagi are hyper-irritable, the stop 

 mechanism acts too soon and breathing becomes shallow and 

 rapid. 



