RESPIRATION. THE MECHANISM OF BREATHING 403 



impulses, regular breathing movements of the nostrils continue. 

 We conclude, therefore, that the activity of the respiratory center, 

 however much it may be capable of modification through sensory 

 nerves, is essentially independent of them. 



What it is that Excites the Respiratory Center. It has long 

 been recognized that the activity of the respiratory center is re- 

 lated to the condition of the blood flowing through it; arterial 

 blood excites it feebly or not at all; venous blood excites it power- 

 fully, and more and more strongly as its venosity increases. The 

 difference between arterial and venous blood is wholly a differ- 

 ence in the relative amounts of oxygen and of carbon dioxid present 

 therein. The question is: Does venous blood owe its ability to 

 stimulate the respiratory center to its low oxygen content or to 

 its high content of carbon dioxid? Experiment has shown that 

 both factors enter somewhat, but that the center is more affected 

 by small changes in the amount of carbon dioxid than by small 

 changes in the amount of oxygen. 



We might look upon carbon dioxid as the main regulator of the 

 respiratory center, and for convenience of description shall do so. 

 As a matter of exactness, however, not carbon dioxid as such but 

 an acid condition of the blood, dependent chiefly on carbon dioxid, 

 determines the activity of the center. Lack of oxygen may pro- 

 duce indirectly the same acid condition that is brought about by 

 excess of carbon dioxid. So we see that we are not entitled to as- 

 sign the control of the center exclusively to carbon dioxid, although 

 that substance determines under ordinary circumstances, its 

 stimulation. 



Why are the Respiratory Discharges Rhythmic? If carbon 

 dioxid is the stimulus for the respiratory center, why does that 

 center act rhythmically? Does the carbon dioxid content of the 

 circulating blood increase and decrease fifteen times or more a 

 minute? The answer to this question is afforded by a simple ex- 

 periment. If in an animal breathing naturally under anesthesia 

 both vagus nerves are cut there is an immediate change in the 

 character of the respirations. From being rapid and shallow they 

 become very deep and take on a much slower rate. Under this 

 condition we may properly assume that the respirations do follow 

 the carbon dioxid content of the blood; the center begins to dis- 

 charge when the blood contains enough carbon dioxid to stimulate 



