following each other oftener than ten times per minute produced merely single initial contractions in the muscle concerned, that is, in a series of stimuli of which the intervals are less than .1 per second, only the first produces response, whereas the following occur in the refractory period, brought about by those preceding, and are, therefore, inoperative. The nerve is fatigued by the quick succession of stimuli. The normal nerve on the contrary invariably responds, as known, to an even more rapid succession of stimuli with a rhythmical excitation corresponding to the number of stimuli and which is manifest in the muscle by a tetanus. This again confirmed the identity of fatigue with the prolonged refractory period, conditioned by the relative want of oxygen. It likewise explained the conditions of the analogous behavior that _Wedensky_[159] had observed in the narcotized nerve, but had neither recognized as manifestation of the prolonged refractory period nor as fatigue. A further advance was made by the investigations of _Thörner_. He placed two nerves of the same frog in a double chamber under completely identical conditions with the exception that one remained in a state of rest, whilst to the other tetanic stimuli were applied. (Figure 35.) If this took place in nitrogen, the irritability of the stimulated nerve invariably sank with much greater velocity than that of the nonstimulated, whereas after an introduction of oxygen, even when the stimulation was continuous, both again recovered. In these experiments of _Thörner_[160] the action current and not the muscle contraction served as indicator. Here the fatigue of the medullated nerve brought about by the deficiency of oxygen during prolonged stimulation is demonstrated in the most obvious manner. (Figure 36.) _Thörner_[161] further succeeded by a continuous stimulation of the nerve in obtaining even in atmospheric air the indications of primary fatigue. The symptoms were exactly the same as those characterizing fatigue of the muscle; the extension of the course of excitation and, as a consequence of this, the appearance of a summation of excitation produced by tetanic currents and a reduction of irritability in response to single stimuli. The form of the curve, resulting from alteration of irritability in fatigue and recovery, likewise shows complete conformity with that of the muscle. (Figure 37.) Finally _Thörner_[162] proved that the nerve, when fatigued by continuous tetanic stimulation in nitrogen, could also partially recover in the latter if the stimulation was interrupted, whereas a complete recovery could not take place unless a supply of oxygen was introduced. (Figure 38.) This fact is in perfect accordance with the relations found by _Verworn_, _Lipschütz_, in fatigue of the nervous centers. It is the expression for the accumulation and removal of fatigue substances, the depressing effect of which _Ranke_[163] first established for the fatigued muscle.