REGULATION OF BREATHING 15 
enormous delicacy towards the slightest changes, up- 
wards or downwards, in the concentration of CO, 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. CO, 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 CO, 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 CO, on the circulation, and he found 
that undue removal of CO, 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 
CO, 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 CO, in a certain concentration in the arterial 
blood is just as necessary to life as, say, the presence 
of oxygen. An environment of CO, is apparently as 
essential as an environment of oxygen. 
