Since I was in the laboratory at Leipzig I have made researches into the changes which cerebral activity exercises on the respiratory movements of men. This is a very complicated problem, as individual differences are great. In the curves which I obtained from some of my colleagues, who kindly placed themselves at my disposition for my observations on respiration, I found that the differences during intense mental work are very considerable. The reason for this must be sought in the variable excitability of the nerve-centres, and especially in the fact that the respiratory mechanism acts in a contrary manner during strong and during weak emotions. I made a few experiments on myself in order to see how the breathing changed, when someone suddenly made a great noise, as, for instance, by firing a gun behind my back while I was reading or absent-minded. I repeated these experiments on dogs, and always found that a deeper, often very deep, inspiration is caused, then there seems to be an arrest of respiration which may last for several seconds, the respiratory movements becoming immediately afterwards more frequent than before. Sometimes in a dog I observed that a shot caused, first, a deep inspiration, then a light expiration and inspiration, while the chest was much expanded; then another deep inspiration like the first, after which the chest emptied itself of the air accumulated in the lungs, and a succession of quicker breaths with more rapid inspirations than usual was drawn. Those who wish to find at once a plausible reason for all phenomena will perhaps say that these deep breaths serve to oxidise and vivify the blood streaming through the lungs, so that in this way the organism may prepare itself for defence. III Let us now see of what parts the respiratory machine consists, and how it puts its force into operation. In man the different parts of this mechanism never act sufficiently in independence of each other to allow of our surprising them separately at work. Only after decapitation has the head been seen to make inspiratory movements although separated from the trunk. Physicians who have had to be present at the execution of the condemned tell of the terrible effect produced when the head of the man rolls, gasping, to the ground, becoming instantaneously of a corpse-like pallor, and disfigured by strange, spasmodic contractions, the eyes rolling irregularly, horribly, for several seconds. The trunk is already motionless, the blood, which at first poured forth in great streams from the arteries of the neck, only spurts feebly and at intervals, still obeying the weakening pulsations of the heart. The eyes at last turn upwards, but life is not yet quite extinct, the mouth still opens from time to time as though with a great effort. The inspirations which at first distended the nostrils and opened wide the mouth soon become less distinct and less frequent, until at last they die away altogether.