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The alveolar wall on contact with air can be compared with a biological air-liquide interface. As with all interfaces, there are therefore superficial forces which tend to reduce the surface to a minimum. In the case of a pulmonary alveolus with a spherical surface, these forces are at the origin of an internal pressure excess dependent on the radius of the alveolus and on the superficial tension related to the nature of the interface. Owing to the disparity in the alveolar radii, under these conditions the smaller alveoli would collapse to the benefit of a larger one, the pressure being lower and lower in the latter. In addition, at any time in the respiratory cycles, this surpression must be negligible in order to avoid rupture of the equilibrium of the forces exerted on the alveolar wall. Consequently, it is necessary that this air-alveolar wall interface should have a superficial tension on the one hand variable with the surface, and on the other hand always very low. Owing to the demonstration of large concentrations of phospholipid in this area, it can be thought that a superficial film is substituted at the air-biological liquid interface and owing to this fact effectively has superficial properties necessary for alveolar stability. The "surface-tensio-active" effect of this film lead to the giving of the name of "surfactant" to these constituents as a whole.

The cerebral metabolic exploration requires a precises measurement of the arterio-venous differences and this, for each metabolite. As far as oxygen is concerned, there are many techniques to measure the blood contents (CaO2 - CvO2). Direct methods give accurate results but their high technicality makes it difficult to adapt them to the increasing medical demand. The authors report the principle and the technique of the indirect method based upon the measurement of the total hemoglobin concentration and the saturation in oxygen. They recall the possible causes of errors, the main of which - induced by an insufficient homogenization of the sample - is (by the way) to be found in both groups of methods although to a lesser degree in the indirect method. Therefore, the latter, that all normally equipped laboratories can carry into practice, seems quite adapted to the "hospital demand" and its validity confirmed by an experience that now belongs to the past.

Following some preliminary remarks on the role of the surfactant in pulmonary physiology and physiopathology, a physiochemical study of the lungs of 26 infants who died in the neonatal period is reported. Two techniques for measuring the decrease in surface tension are set out. The results in the first case (tensiometric method) allow an explanation of the physiopathological aspect of respiratory distress; in the second case (manometric method) the diagrams obtained are characteristic of different clinical states (normal subjects; subjects born prematurely without any hyaline membrane etc...). Furthermore, the second method gives rise to certain hypotheses as to the physiochemical structure of the substances responsible for the tensio-active properties of the surfactant.