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This paper describes two allied methods for the determination of the rate of blood flow which have yielded consistently reproducible results in untrained subjects. The data obtained in forty experiments on twenty-one normal resting individuals are presented in tabular form. The first method is suitable for use with subjects having practically any type of pathology but in the resting condition only. The second method is inapplicable when the subject has a pulmonary lesion preventing ventilation of part of the alveoli or a cardiac defect permitting mixture of arterial and venous blood. With these restrictions, however, this method should be useful as it can be carried out in a short time, with a high degree of accuracy and under varying conditions of activity.[ETHOD I Several investigators have studied the circulation rate, determining the gases in the mixed venous blood by some procedure in which the lungs are used as an aerotonometer, and those in the arterial blood indirectly from the alveolar air or directly by analysis of blood obtained by arterial puncture. Burwell and Robinson, in a contemporary publication (1) have reviewed the literature of the subject and a complete bibliography will not be given here.The method of Christianson, Douglas, and Haldane (2) is difficult to apply to patients but, in suitable subjects, should yield results of the correct order of magnitude. It consists in the inhalation of. a mixture of CO2 and air (oxygen was used instead of air in a few ex-*The expenses of this research were defrayed in part by the Proctor Fund and by the Tutorial Fund of Harvard University.

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