In 1876 _Marey_[119] found that the irritability of the heart in response to artificial stimulation was greatly reduced during the systole, and that recovery took place during the following diastole. (Figure 29.) This fact was already apparent from the observations made by _Bowditch_[120] and _Kronecker_,[121] that by stimulation of the isolated frog’s heart with single induction shocks, an artificial systole can only be produced with certainty when the stimuli succeed each other at certain intervals, which must be the longer as the strength of the stimulation is weaker. _Marey_ calls this period of reduced irritability “_phase réfractaire_” of the heart. The refractory period of the heart has been made the subject of a great number of investigations, especially by _Engelmann_ and his pupils. It was _Engelmann_[122] especially who determined more exactly the duration of the course of the refractory period. He found, namely, that irritability disappears immediately before each systole and reappears shortly before the beginning of the diastole, and again reaches its original height at the end of the diastole. For a long time, however, this refractory period was looked upon as a special peculiarity of the heart. It was not until _Broca_ and _Richet_,[123] twenty years after _Marey’s_ investigations, discovered an analogous refractory period for the motor centers of the cerebral cortex of the dog. They first made this observation on a dog affected with chorea, in which the choreic movements rhythmically occurred in intervals of one second. They found that after each movement electrical stimulation of the cortex remained without result for about .5 seconds. During the next .25 seconds stimulation was followed by a weak response and it was not until the last .25 seconds before the next movement that a strong effect was produced. They also found in the normal dog a refractory period after every artificial stimulation equal to .1 second, so that the number of contractions brought about by rhythmical electrical stimulation were only ten per second. Following this, numerous other investigations of the refractory period have been made on the central nervous system. _Zwaardemaker_[124] and _Lans_ have observed a refractory period in the eyelid reflex of the human being which, on stimulation of the optic nerve, amounts to about .5–1 second; on the stimulation of the trigeminus produced by blowing on the cornea on the other hand, it is somewhat shorter, less than .25 seconds. _Zwaardemaker_[125] also was able to demonstrate an analogous refractory period for the swallowing reflex of the cat. Further a refractory period was found and closely analyzed by _Verworn_[126] for the reflexes in the spinal cord of the strychninized frog. _Dodge_[127] found a refractory period in the knee jerk reflex of man. _Gotch_ and _Burch_[128] showed, by two induction shocks following each other in quick succession, a