INNERVATION OF THE RESPIRATORY MOVEMENTS. 705 



center, but this reaction is nothing like so sensitive as that due to 

 increase of carbon dioxid. The oxygen pressure must be reduced to 

 a very noticeable extent before accelerated respirations are ob- 

 tained. It has been suggested that the effect of reduced oxygen 

 pressure upon the center is, in reality, simply an acid effect. The 

 acids formed in the body as a result of functional activity are usu- 

 ally supposed to be removed by oxidation. If the supply of oxygen 

 is markedly deficient, this removal may be incomplete with a conse- 

 quent increase in the fixed acids and hydrogen-ion concentration 

 of the blood. 



This explanation enables us to understand also some interest^ 

 ing results of the effect of breathing oxygen. When one holds his 

 breath the carbon dioxid tension in the blood increases, and event- 

 ually the stimulus becomes so strong that respirations ensue in 

 spite of the strongest effort to inhibit them. This "breaking point" 

 is reached* in 23 to 77 seconds when the carbon dioxid in the alve- 

 oli of the lungs has attained a concentration of 6.2 to 7.5 per cent., 

 and the oxygen is reduced to 9 to 11 per cent. If before holding 

 the breath the lungs are filled with oxygen by taking several breaths 

 of the pure gas, the breaking point may be prolonged to as much as 

 160 seconds, and one observer (Vernon) reports that if the lungs 

 are first thoroughly aerated by forced breathing, so as to wash out 

 the carbon dioxid in the alveoli, and at the end pure oxygen is 

 breathed in, the breaking point may be deferred as long as eight 

 minutes. Evidently, therefore, an accumulation of carbon dioxid 

 in the blood, as indicated by the composition of the alveolar air, 

 is less efficient as a stimulus to the center when an adequate supply 

 of oxygen is provided, and this fact may be explained on the hy- 

 pothesis that the oxygen prevents the accumulation of the acid 

 products of metabolism. 



A further problem in connection with the action of the respiratory center 

 is the cause of its rhythmic activity. The underlying mechanism of this 

 rhythmicity is obscure, but it is possible that in this instance, as in the case of 

 the heart-muscle, the property is dependent in some way upon the normal bal- 

 ance of calcium, potassium, and sodium in the blood. Variations in the con- 

 centration of these elements cause marked changes in the rhythmicity. f 



The Cause of the First Respiratory Movement. The mam- 

 malian fetus under normal conditions makes no respiratory move- 

 ments while in utero. After birth and the interruption of the pla- 

 cental circulation the first breath is taken. The cause of this 

 sudden awakening to activity on the part of the respiratory center 

 must be closely connected, if not identical with, the cause of the 



* Hill and Flack, "Journal of Physiology," 37, 77, 1908. 

 t Hooker, "American Journal of Physiology," 31, 1913, "Proceedings of 

 the American Physiological Society." 



45 



