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In a recent series of papers Main has dealt with the problem of the fall in the tension of the carbon dioxide in the alveolar air which occurs on the assumption of the erect postur,e. In 1937 he suggested that it was due to an increase in pulmonaiy ventilation. Hitchcock & Ferguson [1938] disagreed with this and pointed out that the phenomenon took place without an increase in ventilation of the lungs. In two of the latter's subjects there was both a fall in the pulmonary ventilation and in the alveolar C02 tension on standing. In 1941 Main defended himself by applying a different interpretation to the term 'over-ventilation'. He pointed out that if the pulmonary ventilation was reduced, and at the same time the volume of blood circulating in the lungs was less, this latter volume could be over-ventilated with a consequent fall in CO2 tension. That Main was, however, mainly concerned with ventilation and the factors affecting this, and not with circulatory phenomena, can be judged from the theme of his papers. In 1940, his opening sentence is: 'The cause of the overventilation in the erect posture is unknown', and he proceeds to investigate whether the fall in pressure within the carotid sinus which occurs on standing is responsible for the increase in breathing. He gives vasopressor drugs to see whether the pressure changes they produce will influence alveolar CO2 tension by their effect on pulmonary vent-ilation. He concludes that the respiratory stimulant is a 'relative cerebral anaemia'. These views are similar to those expressed by Turner [1927], who states that the very moderate increases in respiratory activity which occur on standing can be very easily explained as the result of carotid sinus reflexes and diminished cerebral circulation.The alteration in vital capacity which was pointed out by Hill [1895] has been brought forward as a factor influencing respiration. Bohr [1907] agrees that the vital capacity increases on standing, and Brunton [1908] points out that since the diaphragm has to raise and lower the viscera when one is recumbent, but merely to move them horizontally when upright, this is the key to the limitations which posture places on breathing. Christie & Beams [1922] found from the average of 290 normal subjects that the vital capacity decreases by

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