INNERVATION OF THE RESPIRATORY MOVEMENTS. 695 



tends to reduce the carbon dioxid pressure. In correspondence 

 with this conclusion it is found that in acidosis caused by an increase 

 in the fixed acids of the blood there is a noticeable diminution in the 

 carbon dioxid pressure in the alveolar air. In fact, diminished car- 

 bon dioxid pressure in the alveolar air has been considered as one 

 of the diagnostic indications of a condition of acidosis.* Lack of 

 oxygen, as we have seen, may also stimulate the 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 obtained. 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 usually 

 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 reachedt 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 sufficient oxygon 

 is provided, and this fact may be explained on the hypothesis that 

 an adequate supply of oxygen by oxidizing the fixed acids pro- 

 duced in metabolism prevents their accumulation in the blood. 



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 



* Marriott and Howland. 



t Hill and Flack, "Journal of Physiology," 37, 77, 190S. 



