256 



Comparative Animal Physiology 



plays a part in maintaining the mild muscular tone during eupneic breathing, 

 but the mechanism is largely one of emergency control during severe anoxia or 

 deep anesthesia in which the respiratory center becomes insensitive to increased 

 carbon dioxide tensions. Although chemoreceptive reactions are not to be 

 minimized, we would agree with Schmidt'" -^ that "the reflexes probably do 

 not play an important part in respiratory control under conditions of quiet 

 breathing at sea level but they become much more important under abnormal 

 or emergency conditions in which the blood pO^. or pH falls or the blood pCOi- 

 rises sufficiendy to affect these relatively insensitive but remarkably rugged 

 structures." The oxygen factor as a stimulant may be regarded as negligible 

 at high oxygen tensions and the carbon dioxide negligible at low oxygen 

 pressures.-"" Carbon dioxide increase and oxygen lack summate in their stimu- 



Fig. 58. Influence of O2 and CO:., tensions on type of respiration of the yarrow, a 

 tropical fish. Points designated by circles indicate aquatic respiration, crosses show air- 

 breathing conditions, and solid dots the intermediate stage of transition when either or 

 both may occur. From Willmer."'' 



lating eft'ects as determined by their combined effect on the "breaking point" 

 in breath holding.-*"'' Respiratory adaptability to unusual conditions was shown 

 by Schneider,'"^ who demonstrated that human breath-holding time could be 

 extended to 14-15 minutes after forced breathing and oxygen ventilation. 



In summary, carbon dioxide in the blood is the normal and delicately con- 

 trolling factor in higher animals and acts primarily on the respiratory center 

 itself; whereas oxygen lack is more of an emergency condition, although acting 

 mainly through the chemoreceptive mechanism of the carotid body after the 

 carbon dioxide stimulation has been decreased. The carbon dioxide mechan- 

 ism is the more rclined and phylogenetically is perhaps the more recently 

 developed. 



