RESPIRATION: THE MECHANISM OF BREATHING 365 



impulses, regular breathing movements of the nostrils continue. 

 It is, therefore, nearly certain that the activity of the respiratory 

 center, however much it may be capable of modification through 

 sensory nerves,., is essentially independent of them. 

 ^"WEafirisrtnat Excites the Respiratory Center. It has long been 

 recognized that the activity of the respiratory center is related to 

 the condition of the bloojj, flowing ^throu^h it; arterial blood ex- 

 cites it feebly or not at all ^venous HoocKexcites it powerfully, and 

 more and more strongly as its venosity increased. Trie difference 

 between arterial and venous blood is wholly a difference in the 

 relative amounts of oxygen and *f 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 the carbon dioxid 

 of venous blood is the source of its stimulating power. We may 

 look upon carbon dioxid, then, as a hormone whose function is the 

 chemical stimulation of the respiratory center. 



Why are the Respiratory Discharges Rhythmicf 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 

 it, and continues its discharge until the aeration of the blood, re- 

 sulting from the inspiration, lowers the carbon dioxid below the 

 point of stimulation. There follows a period of expiration and 

 rest which continues until sufficient carbon dioxid has again ac- 

 cumulated to start the action anew. 



Since with the vagus nerves cut the respirations follow the car- 

 bon dioxid concentration of the blood, but with the nerves intact 

 do not, being much more shallow and rapid, we must determine 

 the influence of the vagus nerves upon the center in order to un- 

 derstand ordinary breathing. It has been shown that the influence 



