RESPIRATION 229 



those of oxygen want : moreover the secretion did not appear if 

 oxygen was breathed along with the CO, until a much higher 

 saturation of the blood with CO was reached. Pure oxygen, as 

 already shown in Chapter VII, provides a certain supply of dis- 

 solved oxygen to the blood independently of the oxygen carried by 

 the haemoglobin, and thus prevents, to a large extent, the oxygen 

 want which would otherwise be caused by the CO. 



Now the oxygen want is in the tissues, and not in the lungs. 

 Hence the stimulus to secretion originates in the tissues. This 

 stimulus is almost certainly something carried by the blood from 

 the oxygen-starved tissues to the lungs or central nervous system. 

 One might perhaps suppose that whenever the respiratory center 

 is excited, nervous impulses pass down secretory fibers in the vagus 

 nerve and excite secretion in the lungs. Lorrain Smith and I tested 

 this hypothesis, and found that when the respiratory center was 

 excited by excess of CO 2 there was not the slightest rise in the 

 arterial oxygen pressure. Hence the secretion has no direct con- 

 nection with the ordinary activity of the center in producing 

 respiratory movements; and the stimulus to secretion is not a 

 hydrogen ion stimulus. 



We also made a series of determinations on man. In view of 

 the results of the mouse experiments we were anxious to work 

 with low percentages of CO ; but if we had used the old method 

 which Lorrain Smith and I had employed, it would have taken 

 so long before equilibrium was reached between the CO in the 

 air and that in the blood that our experiment could hardly have 

 been completed during winter daylight. We therefore adopted 

 the course of quickly absorbing as much CO as would saturate 

 the blood to the desired extent, and then breathing in and out of 

 a small air space, in which the oxygen and CO 2 percentage was 

 kept constant. Under these conditions CO must, of course, be 

 given off into the air of the space, and as this air is breathed again 

 and again, equilibrium between the CO in the air and that in the 

 blood must establish itself very quickly. The method finally 

 adopted was as follows (see Figure 67). 



The subject, wearing a nose clip, breathes through the mouth- 

 piece A, inhaling through the inspiratory valve B, and expiring 

 through the valve C. The expired air passes through a rubber 

 pipe of large caliber to the tin vessel D, which is filled with small 

 fragments of solid caustic soda, and is made of such a size (di- 

 ameter 23 cms., depth 12 cms.) that the whole of the carbonic 

 acid in the expired air is effectively removed. Another rubber 



