HEAD'S PARADOXICAL REFLEX

Cold block of the vagus nerves of rabbits completely prevents conduction in pulmonary stretch fibres at temperatures close to or higher than those required to block the Hering‐Breuer inflation reflex. These temperatures are several degrees higher than those required to block Head's paradoxical reflex. It is concluded that Head's paradoxical reflex cannot be due to pulmonary stretch fibre activity during cold block; that is, cannot be due to cold block of the vagus nerves being more effective for high‐frequency discharges (during lung inflation) than for low‐frequency discharges in pulmonary stretch fibres. This conclusion is supported by a consideration of the pattern of Head's paradoxical reflex, and by a summary of the published evidence that the reflex can be elicited when the nerves are not being cooled at all.