314 RESPIRATION 



impulses, and this fact has been explained by the supposition that it is 

 .stimulated to action by the condition of the blood circulating through it. 

 When the blood becomes more and more venous the action of the center 

 becomes more and more energetic, and if the air is prevented from entering 

 the chest, the respiration in a short time becomes very labored. If the 

 aeration of the blood is much interfered with, not only are the ordinary 

 respiratory muscles employed, but also those muscles of extraordinary in- 

 spiration and expiration which have been previously enumerated. Thus, 

 as the blood becomes more and more venous, and by venous we mean that 

 the blood contains a relatively large amount of carbon dioxide and a dimin- 

 ished amount of oxygen, the action of the medullary center becomes more 

 and more profound. The question has been much debated as to what 

 quality of the venous blood it is which causes this increased activity; whether 

 it is its deficiency of oxygen or its excess of carbon dioxide. It has been 

 answered to some extent by experiments which offer no obstruction to the exit 

 of carbon dioxide, as when an animal is placed in an atmosphere of nitrogen. 

 Under these conditions dyspnea occurs, showing that blood which contains 

 a diminished amount of oxygen stimulates the cells of the respiratory center. 

 On the other hand, if the normal amount of oxygen is supplied while the 

 carbon dioxide of the blood is prevented from escaping and thus allowed to 

 accumulate in the blood, there is also a great increase in the respiratory 

 activity of the center; an excess of carbon dioxide in the blood, flowing 

 through the respiratory center, stimulates the cells to greater activity. It 

 is highly probable, therefore, that the respiratory centers may be stimulated 

 to action both by the absence of sufficient oxygen in the blood circulating 

 in it, and by the presence of an excess of carbon dioxide. 



These facts are particularly well supported by the experiments of Zuntz 

 who varied the oxygen and the carbon-dioxide content of the air breathed, 

 and measured the volume breathed per minute. When the oxygen of the 

 air breathed was reduced by 10 to 50 per cent., the air breathed was increased 

 only slightly, 5 to 10 per cent. When the oxygen of the air was reduced 

 by 60 per cent., the volume of air breathed was increased 30 to 40 per cent., 

 and even more. Other observations show us that the oxygen in the blood 

 in these experiments will fall in much less per cent, than the reduction in 

 the oxygen of the air would lead us to suspect. 



When Zuntz kept the oxygen content of the air about constant, but in- 

 creased the carbon-dioxide content, then the amount of air breathed was 

 greatly increased. Air containing 18.4 per cent, of oxygen and 11.5 per cent, 

 of carbon dioxide caused an increase in the amount breathed per minute 

 from 7.5 liters to 32.5 liters. These experiments indicate that within the 

 limits of its normal variations in blood the carbon dioxide has a much greater 

 influence than oxygen on the irritability of the cells of the respiratory center. 



But this is not all, since it has been observed by Marckwald that the 



