I went to work at once and constructed two apparatus, one for each arm, with the intention of studying the circulation in two parts of the body at the same time. The phenomenon which had most surprised me in my first experiments in Italy was the great instability of the blood-vessels of the hand, in consequence of which it changed in volume under the slightest emotions in the most surprising manner, whether the subject were awake or asleep. A few days after having installed myself in the laboratory in Leipzig, I was making an experiment in a room near that of the professor, my colleague, Professor Luigi Pagliani, helping me in everything with the devotion of a friend. Our first object was to establish the relation between respiration and change of volume in the hands. While Professor Pagliani was standing before the registering apparatus, with his arms in the glass cylinders filled with water, Professor Ludwig walked into the room. Immediately the two pens indicating the volume of the arms, descended, as though a vertical line, ten centimetres in length, were drawn down this page. It was the first time that I had seen such a considerable decrease in the volume of the hand and fore-arm, produced by an apparently slight emotion. Professor Ludwig himself was very much astonished, and, with that affability which made him so beloved by his pupils, took a pen and wrote on the paper at that point where the plethysmograph had marked the disturbance in the circulation caused by his appearance, _Der Löwe kommt_ ('Enter the lion’). III In order to show more clearly the perpetual changes of locality which the blood undergoes, accumulating now in one, now in another part of the body, I constructed a balance of such a size that the beam (made of wood) was sufficiently long and broad to allow of a man’s lying at full length upon it, as may be seen in fig. 2. By means of the weights, _R_, which run along the edge of the couch (moving upon the fulcrum, _E_), it is easy, when the centre of gravity of the body is nearly in the middle of the balance, to keep a man in equilibrium. In order to prevent the balance swaying from side to side at every little oscillation, I had to affix a heavy counterpoise of metal, _I_, which can be moved up or down upon the screw, _G H_, fixed vertically in the middle of the plank, _D C_, and firmly held by the lateral bars, _M L_. [Illustration: FIG. 2. BALANCE FOR THE STUDY OF THE CIRCULATION OF THE BLOOD IN MAN] The centre of gravity of the balance is placed in this way so low down that it no longer sways at every little oscillation, the counterpoise, which moves inversely to the inclination of the balance, by its weight drawing the plank with it, and bringing it again into a horizontal position. I made the balance so sensitive that it oscillated according to the rhythm of respiration.