The mean volume of the extrathoracic respiratory tract in six cadavers was found to be 72 ml (BTPS), (S.D. ±32). Expressed as a fraction of body weight in pounds this amounted to 0.55. The intrathoracic anatomical dead space was measured in three intubated subjects. The mean value was 66 ml (BPTS), (S.D. ±29) or 0.43 times the body weight in pounds. The influence of the position of the jaw on the dead space was studied in the six cadavers and three conscious subjects. Depression of the jaw with flexion of the neck produced a mean decrease in the dead space of 31.4 ml while a protrusion of the jaw with extension of the neck increased the dead space by 39.7 ml. Submitted on July 28, 1958

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When an elastic structure such as the thorax is inflated its elastic tissues are stretched. The recoil force produced by these tissues can be determined by measuring the inflating pressure within the airways. By recording the pressure at different inflation volumes a pressure-volume diagram may be constructed.Rahn, Otis, Chadwick & Fenn (1946) made use of this principle when they studied the elastic properties of the thorax in conscious human subjects by determining the relaxation pressure curve. After the inspiration of different volumes of air, the airway is occluded; the respiratory muscles are then deliberately relaxed and the recoil pressure (relaxation pressure) which then develops is measured. In our experience there is a wide scatter of the individual pressure-volume points on repeated measurements. It would seem therefore that even trained and co-operative subjects are unable to relax their respiratory muscles completely or to the same extent at all times when pressurevolume measurements are being made.Nims, Conner & Comroe (1955) have reported that the compliance of the thorax was lower in anaesthetized patients made apnoeic in various ways than in the same subjects when conscious; they suggested that some inspiratory activity was present during the pressure-volume determinations in the conscious state.The present study of the thoracic compliance in anaesthetized paralysed subjects was undertaken in the belief that the results would represent the true total thoracic compliance, i.e. when uninfluenced by muscle activity, and that reliable data would be obtained for the compliance of the thoracic cage itself about which little has been reported. Our results confirm and extend those of Nims et al. (1955); we have, however, been led to believe that the mechanism of inflation when the thorax is ventilated by positive pressure is different from 1 PHYSIO. CXXXVI

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