Arrangement for an artificial circulation in the frog. A--Accumulator. B--Metronom. C--Mercury key. D--Electromagnetic apparatus for compressing the rubber tube: 1, wire spool with magnet; 2, anchor for the magnet; 3, spiral spring which pulls back the anchor; 4, axis on which the anchor turns; 5, plate for arresting the anchor. E--Vessel containing saline solution. F--Slab of cork with frog. ] The fluid flows from a vessel, E, provided with an outlet tube through a thin rubber tube into a glass canula, which is introduced into the general aorta of the frog, F. The tube is automatically occluded by the rhythmical movement of the armative of an electromagnet, D, produced by a metronome, B. The pressure of the circulating fluid can be readily changed at will by varying the level of the vessel and the frequency of the pulse by the rhythm of the metronome, which makes and breaks the current to the electromagnet.[141] In this way it is possible to artificially replace the normal circulation with satisfactory exactitude and substitute for the blood, circulating in the vessels of the frog, any desired fluid. If the entire quantity of blood of a frog is displaced by a continuous stream of oxygen-free saline solution and a weak strychnine solution is injected with a Pravaz syringe, a violent strychnine tetanus appears after the lapse of a few seconds. (Figure 32, A.) If the artificial circulation with oxygen-free saline solution is now contained in the rhythm of the natural heart beat, the further reactions can then be readily observed. The first long-continued tetanic attack, which can be produced by a slight touch of the skin, is followed by a whole series of tetanic convulsions of prolonged duration, which are repeatedly followed by periods of exhaustion. I wish to emphasize this fact once more, as it appears to me as not without interest for the understanding of the question of reserve substances. [Illustration: Fig. 32. Muscle curve of strychnine tetanus in a frog with artificial oxygen-free circulation. Lower line indicates seconds. Upper line indicates stimulation by induction shocks. A--A single shock produces a long tetanic contraction. B--In a more advanced stage each shock produces a tetanus only of short duration. C--In a still more advanced stage each shock brings about only a single contraction if the stimuli do not succeed each other too rapidly. If they succeed more rapidly, as, for instance, in a faradic current, only the first shock is effective. ]