290 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



the activity of the centre to the larger quantity of carbon dioxide. It 

 was later on shown by Pfliiger and Dohmen 1 that either of these two 

 moments — increased carbon dioxide or diminished oxygen — might serve 

 as a respiratory stimulus. These authors controlled their results by 

 analyses of the blood. 



Where dyspnoea followed inhalation of hydrogen or nitrogen, they 

 found that the blood only differed from normal blood in containing 

 less oxygen. On the other hand, when the animals were made to 

 breathe mixtures of carbon dioxide and oxygen, the blood contained 

 a normal amount of oxygen, but an excess of carbon dioxide. In 

 both these cases dyspnoea was produced, although in the one instance 

 deficiency of oxygen, and in the other excess of carbon dioxide, were the 

 only deviations from the normal. It is in the first stages alone that the 

 results of these two experiments are similar. If the inhalation of the 

 abnormal gaseous mixtures be continued, it is found that deprivation of 

 oxygen leads to ever-increasing dyspnoea, and finally general convulsions, 

 the animal dying of asphyxia. When, on the other hand, the animal 

 breathes a mixture of carbon dioxide and oxygen, the respiration, which 

 is deepened and slightly quickened at first, undergoes very little further 

 alteration. The animal can continue to breathe the mixture for a 

 considerable time without apparent ill effects ; then respirations gradually 

 become slower, the animal passes into a deep sleep, and finally dies 

 without convulsions, as under the influence of a narcotic poison. 



Bernstein z attempted to show that carbon dioxide dyspnoea differed 

 from that produced by absence of oxygen, in the fact that carbon 

 dioxide chiefly augmented expiration, while absence of oxygen increased 

 inspiration. More careful experiments by Gad 3 have proved that this 

 is not the case, but that in each instance the dyspnoea is mainly 

 inspiratory. 



Some physiologists have found a difficulty in conceiving how the 

 absence of a substance (oxygen) could act as a stimulus. It has therefore 

 been suggested that the exciting agent was probably represented by 

 such " reducing substances " as have been described by Schmidt i and 

 Pfliiger 5 as occurring in the blood of asphyxiated animals. 



According to the theory of Pfliiger, metabolic changes are constantly 

 occurring in living cells, including nerve cells, leading to the formation 

 of molecules, or groups of molecules, with a high affinity for oxygen. It 

 is these intracellular reducing substances which combine in the first 

 instance with the oxygen leaving the vessels, or are able to tear away 

 the oxygen from such substances as methylene blue, as in Ehrlich's 

 experiments. 6 It is possible that these bodies, when they meet with 

 insufficient oxygen for their saturation, act as excitants on the nerve 

 cells ; so that the normal activity of these cells is maintained by a con- 

 dition of auto-stimulation, the stimulus being supplied by their own 

 products of metabolism. On the other hand, when oxygen is supplied 

 in excess, the affinities of the reducing substances are satisfied. No 



1 Untersuch a. d. physiol. Lab. zu Bonn. 1875, S. 83. 

 - Arch./. Physiol, Leipzig., 1882, S. 313. 



a Ibid., 1886, S. 388 {Verhandl. d. Berl. phys. Gesellseh.), confirmed by Max 

 Rosenthal, ibid., 1886, Suppl., S. 248. 



4 Arb. a. d. physiol. Anst. zu Leipzig, 1867. 



5 Arch. f. d. ges. Physiol., Bonn, 1868, Bd. i. S. 61. 



6 " Das Satierstoffbedurfniss des Organismus," Berlin, 1885. 



