RESPIRATION 13 



lasts on after the distention ceases, and from this supposed fact 

 the further inference was drawn that repeated distention of the 

 lungs produces a summed vagus effect resulting in vagus aprioea 

 after the distentions have ceased. Thus the same procedure that 

 causes chemical apnoea seemed to produce also vagus apnoea, 

 and the two kinds of apnoea could hardly be distinguished in 

 practice. Moreover hyperpnoea due to any chemical cause such as 

 want of oxygen or excess of CO 2 must apparently tend to be 

 prevented by the production of vagus apnoea due to repeated 

 distentions of the lungs. The two processes by which the breathing 

 appeared to be regulated acted, therefore, in opposite directions. 



As regards the chemical stimuli acting on the respiratory center, 

 it remains to consider the further evidence as to the relative im- 

 portance of want of oxygen and excess of CO 2 ; also whether other 

 chemical stimuli act on the center. In 1885 Miescher 21 showed 

 by experiments on man that a given small increase in the per- 

 centage of CO 2 in air affects the breathing considerably, while a 

 corresponding diminution in the oxygen percentage has no such 

 effect. He was thus led to the conclusion that it is the CO 2 per- 

 centage in the air of the lungs that ordinarily determines the 

 chemical regulation of breathing, and not the oxygen percentage. 

 Thus CO 2 protects the body from want of oxygen so long as 

 ordinary air is breathed. It will be seen in the sequel how rela- 

 tively correct this general view of Miescher's was, although he 

 maintained the existence of vagus apnoea and thus shared in the 

 mistakes of his time. 



In 1888 Geppert and Zuntz 22 published the results of a very 

 careful series of experiments on the effects of muscular work (pro- 

 duced by tetanizing the hind limbs of an animal after section of the 

 spinal cord) on respiration. After bringing forward new evidence 

 that it is the blood which carries the stimulus for increased breath- 

 ing to the respiratory center they showed that during the work 

 the proportion of CO 2 in the blood was greatly diminished, and 

 that there was also a slight increase in the oxygen percentage of 

 the blood. Hence, they argued, it is neither increase in CO 2 per- 

 centage nor diminution in oxygen percentage that causes the 

 hyperpnoea accompanying muscular exertion. They believed that 

 it is some acid substance produced in the muscles, and pointed out 

 that Walter had found that the breathing is much increased in 

 poisoning by acids. 



21 Miescher, Arch. f. (Anat. uJ) Physiol., p. 355, 1885. 



32 Geppert and Zuntz, Pfltiger's Archiv, XLII, pp. 195, 209, 1888. 



