276 RESPIRATION 



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 inspiration and expira- 

 tion 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 diminished 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 ex- 

 periments 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, there- 

 fore, 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 increased the amount breathed per minute from 7.5 liters 

 to 32.5 liters. These experiments indicate that within the limits of the 

 normal variations in blood the carbon dioxide has a much greater influ- 

 ence 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 

 medullary center is capable of acting for some time in the absence of any 

 circulation, and after excessive bleeding. The view taken by this author 

 with regard to the action of the center is as follows: The respiratory center 



