Fig. 31.
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.
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.
If we assume that at the moment when the entire amount of blood is removed from the vascular system, no oxygen remains in the cells of the spinal cord and muscle, then disintegration of the living substance could from this instant take place exclusively anoxydatively, and there would be no further oxydative breaking down into carbon dioxide and water. The energy production compared in equal number of molecules, taking the figures of Lesser for the fermentation of sugar, would approximately amount to about 3.8 per cent. of that of the energy production in the oxydative disintegration of dextrose into carbon dioxide and water. In reality, however, the tetanic convulsions are at first exactly as violent as in the frog with a normal circulation. There simply remains the assumption, therefore, that either the disintegration as soon as it becomes anoxydative involves relatively greater number of molecules than would be the case if it were oxydative in nature, or to suppose that even after the complete displacement of the blood a certain, though relatively small, amount of oxygen is present in the cells which for a short time suffices for the taking place of oxydative disintegration and with this an almost maximal production of energy which naturally decreases as the oxygen is consumed. It seems to me that the latter supposition contains more probability than the first. To return, however, from this observation to a further consideration of the animal we are studying, we see how the complete tetanic convulsions in the refractory period which we assumed to be .1 second are gradually transformed into incomplete tetanus. After a time the tetanic convulsions become shorter after each stimulus (Figure [32], B) and permit us to distinguish their individual movements, even though the latter at first succeed each other still very rapidly. Gradually this incomplete tetanic convulsion assumes the form of a short series of individual contractions, distinctly separated from each other and soon a stage is reached in which each reaction to a peripheral stimulus consists merely in a single contraction. (Figure [32], C.) The refractory period is, however, even now less than a second. Nevertheless, with a further continuation of the experiment, the refractory period becomes more and more prolonged, so that stimuli succeeding each other at intervals of less than a second are without effect. It is possible at this stage, as Tiedemann[142] did, to graphically record the reactions. He severed the sciatic nerve on one side and stimulated its central stump, at the same time connecting the triceps with a writing lever. It is then found that when the single induction shocks follow each other at intervals of a second or more every stimulus produces a contraction, but that on the contrary only the first stimulus of a rhythmical series is operative and all those succeeding ineffectual, if the stimuli follow each other at shorter intervals. The refractory period becomes, however, more and more prolonged. The rhythm of the stimulus must become continually slower if each individual stimulus is to remain effective. If the rhythm is even slightly too rapid only the first few stimuli of a rhythmical series are effective and this with decreasing response and later no contraction at all is observed. With a further continuance of the experiment, the stimuli are only effective when following each other at long intervals. It is necessary that a period of recovery lasting several seconds must take place before the following stimulus can meet with response. (Figure [33].) The refractory period can gradually be prolonged for the space of a minute or longer, until finally irritability does not reappear at all, and even the strongest stimuli fail to produce the least contraction. The continuous manner in which the refractory period is, in the absence of oxygen, more and more prolonged until eventually a prolonged state of nonirritability is developed, can be better followed by observing the experiment than when described in words. If at this stage instead of the oxygen-free saline solution, defibered blood of the ox shaken in air or a saline solution saturated with oxygen is circulated in the frog, restitution is often within a few minutes so complete that tetanic attacks are once more produced by a single stimulus, that is, the refractory period has from being practically nil returned to the normal. This experiment can be repeated several times on the same animal. It is invariably found that the refractory period is prolonged by the withdrawal of oxygen and shortened with a renewed supply.
Fig. 33.
Development of the refractory period in the spinal cord of a strychninized frog. Lower line indicates seconds; upper line stimuli. Of a series of stimuli only the first ones are operative with decreasing effect.