Fig. 10.
Rhizoplasma kaiseri. A—Under normal conditions. B—In an atmosphere of pure hydrogen.
During a sojourn at the Red Sea in 1894–95 I was able to establish this dependence in the single-celled organism, the Rhizoplasma Kaiseri, a large naked orange-colored rhizopod. (Figure [10], A.) Mechanical stimulation, which under normal vital conditions of these organisms brings about contraction in the long-branched pseudopods, becomes ineffective with a cessation of the movement of protoplasm, when oxygen is removed and is replaced by a stream of hydrogen. (Figure [10], B.) With renewed introduction of oxygen there is a return of the protoplasmic movement and entire recovery takes place.
This dependence of irritability upon oxygen is most clearly demonstrated in the nerve centers. For this purpose I have employed the spinal cord of the frog.[60] A canula is introduced and fixed into the aorta of the animal and the blood is replaced by a current of oxygen-free saline solution. The centers of the spinal cord are thereby wholly isolated from the supply of oxygen. The indicator for the irritability here used is reflex excitation from the skin to the gastrocnemius, or better, stimulation of the central stump of the sciatic nerve with single induction shocks, bringing about reflex response of the triceps. The reflex may be considerably augmented by increasing the reflex excitability of the spinal cord by poisoning the animal with strychnine. On testing the reflex excitability at the beginning of the experiment it will be found that the reaction to each individual stimulus consists, in consequence of the strychnine poisoning, of a long-continued maximal tetanus. The longer the deficiency of oxygen continues, the briefer become the tetanic reflex contractions following a single stimulus. Soon reflex tetanic responses are merely short single contractions, which decrease more and more with the continuance of oxygen deficiency. Finally, the same stimuli which previously produced strong tetanic contractions of long duration are altogether without effect. Although by increasing the intensity of stimulation brief contractions can again be brought about, irritability decreases more and more, until at last even the strongest stimuli remain without result. If the oxygen-free saline solution is now replaced by one saturated with oxygen, or blood of the ox, rendered arterial, the excitability returns within a few minutes and soon reaches the maximal height which it possessed under the influence of the strychnine poison. Even the weakest single stimuli now again produce tetanus. The same process reoccurs, if the fluid used for transfusion containing oxygen is again replaced by an oxygen-free saline solution. In this way, by repeated change of the perfusing fluid, we can demonstrate in the most positive manner this alteration in irritability, the result of the alternate presence and removal of oxygen. This is perhaps the best example of the close dependence of irritability on oxygen.
This same fact can be observed with equal clearness in the nerve. At my suggestion H. v. Baeyer[61] showed as the result of investigations made in the Göttingen laboratory the dependence of irritability of the nerve upon oxygen for the first time. By employing as the method the ascertainment of the threshold of stimulation I then made a closer study of the alterations in irritability during asphyxiation. These observations were soon after continued by Fröhlich.[62] The method is as follows: the nerve of a nerve-muscle preparation of the frog is drawn through a glass chamber which is made completely air-tight and containing platinum electrodes. The air in the chamber is then displaced by a stream of pure nitrogen. (Figure [11].) On testing that part of the nerve situated within the glass chamber with single break induction shocks it can be observed that its irritability, measured by the threshold of stimulation for muscle contraction, decreases more and more, until after the lapse of some hours, the stimulation required is so strong as to reach the region of the “Stromschleifengrenze.” If in place of the stream of nitrogen, air or pure oxygen is now allowed to flow through the chamber, the nerve recovers almost instantaneously. Within the space of a minute its irritability has risen again to its full height and the same experiment, with the same result, can be repeated. Finally, as Fillié[63] has shown, the like result is obtained when the nerve is asphyxiated in a fluid medium.
Fig. 11.
Arrangement for asphyxiating the nerve. A—Gasometer containing pure nitrogen. B and B1—Vessels for washing the gas. C—Ether chamber for eventual experiments with narcosis. D, D1 and E—Glass faucets. F—Moist chamber. G—Asphyxiation chamber. H and H1—Two pairs of electrodes over which the nerve is laid. I—Nerve muscle preparation.
All these facts, the number of which indeed could be increased greatly for other aërobic forms, suffice to establish the fundamental importance of oxygen to the maintenance of irritability of living substance. Oxygen is of greatest importance for a high degree of irritability in all aërobic organisms. All living systems which are characterized by a great capability of activity and evince strong responses under the influence of stimulation, such as the vertebrates and insects, are necessarily aërobic, whereas the living organisms of pronounced anaërobic character, as some bacteria, yeast cells, parasitic organisms, etc., manifest on the average much less capability of activity.
Finally, to briefly summarize the foregoing, the following picture presents itself of disintegration produced by a momentarily acting stimulus. It is immaterial how the stimulus produces an excitating effect in the given case, whether through changes in the ion concentration of the living system, by increase of intramolecular atomic movement or in any other manner, it invariably accelerates the disintegration of the complex molecules concerned in functional metabolism, the nature of which varies in the special cases. In the great majority of instances nitrogen-free organic combinations serve as material for the functional constituent members of metabolic processes. In the anaërobic organisms this decomposition takes place anoxydatively with the coöperation of enzymic processes, and as larger fragments generally result from the disintegration of the complex molecule, the production of energy is accordingly smaller. The disintegration of aërobic organisms, on the other hand, occurs in the form of an oxydative splitting up of the complex molecules into carbon dioxide and water so that the production of energy attains a high value. The details concerning the manner in which the individual stages of this decomposition take place and the interactions by which its end products are reached is at present beyond our knowledge. It would be a mistake to generalize in this connection from the behavior of certain groups of organisms. The assumption that under certain conditions the disintegration occurs in two phases, the splitting up resulting from enzymic action of the complex molecule into larger fragments, followed by an oxydative splitting up of these into carbon dioxide and water, can in no case as yet be justifiably applied to all conditions and all aërobic organisms. This is more or less the impression which we derive of the functional excitation process as seen today.