REGULATION OF BREATHING 15 



enormous delicacy towards the slightest changes, up- 

 wards or downwards, in the concentration of C0 2 in 

 the alveolar air in contact with the arterial blood 

 which supplies the centre. 



It is of the highest significance that a slight change 

 in the downwards direction is sufficient to suspend 

 natural or involuntary breathing. C0 2 was formerly 

 regarded as merely a "waste product," the getting rid 

 of which as rapidly and completely as possible could 

 only be a physiological advantage. It has turned out, 

 however, that the presence of a certain concentration 

 of C0 2 is essential to the continuance of breathing. 

 This brings us at once into connection with a series 

 of investigations independently initiated by Professor 

 Yandell Henderson of Yale, and afterwards carried 

 on side by side with the Oxford investigations. His 

 work was at first concerned mainly with the effects of 

 concentration of C0 2 on the circulation, and he found 

 that undue removal of C0 2 from the blood has the 

 most disastrous effects on the circulation, producing 

 symptoms similar to those observed in the surgical 

 condition known as "shock." He found that when 

 C0 2 is removed from the body in undue quantity by 

 excessive artificial ventilation of the lungs, the heart 

 and circulation gradually fail, and death results. To 

 this subject I will return later; but I am referring to 

 it now in order to emphasise the point that the pres- 

 ence of C0 2 in a certain concentration in the arterial 

 blood is just as necessary to life as, say, the presence 

 of oxygen. An environment of C0 2 is apparently as 

 essential as an environment of oxygen. 



