I have in my laboratory a dog which was of service to me in a few studies on fatigue. He is such a good animal that for two years I have kept him, together with two other dogs of which I have grown fond, and which, like old friends, shall always stay with me, unless some dog-lover comes to beg them from me, as so often happens with good, faithful dogs, that only in the physiological laboratories can escape from the certain and cruel death to which the Corporation condemns them. As he is a quiet dog, it occurred to me one day to try the effect of a violent noise upon him. I made use of a little instrument called a _cardiograph_, because it transmits the heart-beats to a lever which traces them on a cylinder covered with smoked paper. I applied this instrument, which is about the size of a half-crown, on the place where the heart beats between the ribs, fixing it by means of an elastic band fastened round the thorax. At first it wrote the curve of the cardiac pulsations represented in fig. 3, which is reproduced by means of photozincography. I regret having to present the reader with more curves, but when one is able to see what the heart itself writes, it would be unpardonable to try to translate its characteristic language into words. Besides, it is not difficult to understand these curves. The line _T_ signifies the time; it is written by an electric clock which raises a pen every second and marks a tooth. It is, so to speak, a controlling line, indispensable in graphic studies by which one wishes to learn with the greatest exactness the changes which the frequency of the pulse undergoes. In the line _T_ eighteen seconds are marked, and the heart-beats registered in the same time in line _A_ amount to twenty-nine. If I had applied the cardiograph to the thorax of a man I should have obtained a similar curve with fewer pulsations. At each beat of the heart the pen rises and falls rapidly, and then writes below a trembling line during which the heart does not beat. As the thorax rises and expands during inspiration, the pen resting on the ribs must likewise rise, therefore the three or four pulsations taking place during the expansion of the thorax are marked successively higher, sinking again with the commencement of expiration, thus forming waves, as it were. From this curve, traced while the animal was tranquil, we see that its heart, as indeed is also the case in man, beats more frequently during inspiration than during expiration, the heart-beats being nearer to each other in the ascending portion of the curve, and further apart in the lower portion which corresponds to the end of each expiration.