1. A technique for determining the respiratory response to carbon dioxide on the Peabody principle is described.

2. The relation between minute volume of total pulmonary ventilation and percentage of carbon dioxide in the inspired air can be expressed by a simple mathematical formula, viz. Y = K + abz, in which Y is the ventilation rate, X is the CO2 content of the inspired air, and K, a, and b are constants characteristic for the individual.

3. The respiratory response to carbon dioxide as expressed by the total pulmonary ventilation is slightly greater at high oxygen percentages (90 per cent ±) than at normal oxygen percentages in the inspired air.

4. Respiratory fatigue may consist of two elements—one nervous, manifesting itself in increased excitability of the center and a more marked response when the demand for pulmonary ventilation is small, the other muscular and involving an inability to respond when the demand for pulmonary ventilation is great.

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