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. [60] _Max Verworn_: “Ermüdung Erschöpfung und Erholung der nervösen Centren des Rückenmarks. Ein Beitrag zur Kenntniss der Lebensvorgänge in den Neuronen.” Archiv. f. Anat. u. Physiologie. physiol. Abteil. 1900 Suppl. The same: “Ermüdung und Erholung.” In Berliner Klin. Wochenschrift 1901.