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. [139] _Max Verworn_: “Allgemeine Physiologie.” V. Auflage. Jena 1909. [140] _Max Verworn_: “Ermüdung Erschöpfung und Erholung der nervösen Centra des Rückenmarks.” Arch. f. Anat. u. Physiol. physiol. Abt. Suppl. 1900. The same: Ermüdung und Erholung. Berliner Klin. Wochenschrift 1901. [141] As I have not yet described this method elsewhere the above figure will suffice for demonstration. [142] _Tiedemann_: “Untersuchungen über das absolute Refractäerstadium and die Hemmungsvorgaenge im Rückenmark des Strychninfrosches.” Zeitschrift f. allgem. Physiologie Bd. X, 1910. [Illustration: 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. ]