Vital capacity is measured by the largest expiration following the deepest inspiration. Until recently this physical feature of the respiratory functions received little serious consideration except as a physiologic experiment. Peabody has significantly remarked that those who, during the last few years, have been studying the function of respiration have devoted their attention largely to the changes in the blood gases and have paid little attention to the mechanical aspect of pulmonary ventilation. Variations in the gaseous content of the blood are of course of the utmost importance, and they affect the stimulus to respiration; but at the same time, Peabody adds, one cannot neglect the response on the part of the lungs to this stimulus. This response manifests itself in the ventilatory function of the lungs, which promotes the proper exchange of gases through the cells of the alveoli.
Since vital capacity is an indication of the competence of