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. [61] _H. v. Baeyer_: “Das Sauerstoffbedürfniss des Nerven.” Zeitschrift f. allgemeine Physiologie Bd. II, 1903. [62] _Fr. W. Fröhlich_: “Das Sauerstoffbedürfniss des Nerven.” Zeitschrift f. allgem. Physiologie Bd. III, 1904. [63] _H. Fillié_: “Studien über die Erstickung des Nerven in Flüsigkeiten.” Zeitschrift f. allgemeine Physiologie, Bd. VIII, 1908. [Illustration: Fig. 11. Arrangement for asphyxiating the nerve. A--Gasometer containing pure nitrogen. B and B_{1}--Vessels for washing the gas. C--Ether chamber for eventual experiments with narcosis. D, D_{1} and E--Glass faucets. F--Moist chamber. G--Asphyxiation chamber. H and H_{1}--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.