From the information here gained on the nature and origin of the refractory period the conclusion must inevitably be drawn that in all living substance there must exist, directly following an excitation, a period of time in which its irritability is reduced, that is, under proper conditions a refractory period can be demonstrated for every living organism. Every living system possessing irritability undergoes a period of reduced irritability at the time of and subsequent to every excitation, for every excitation momentarily decreases the amount of products capable of disintegration and increases the disintegration products in the unit of space. As restitution involves time, a stimulus occurring in the phase preceding complete restitution cannot break down the same quantity of molecules as would be the case after the establishment of complete restitution, that is, the response is weaker, the irritability is decreased. The refractory period during and subsequent to excitation is as much a general property of the living substance as irritability and metabolic self-regulation. This conclusion appears so self-evident that it would seem hardly to call for emphasis were it not that even at the present time the view is still widely held that the refractory period is a special characteristic of certain forms of living substance. This assumption is explained on the one hand by the fact that our information concerning the refractory period is still of comparatively recent date and that few physiologists are in the habit of connecting special observations with general physiological conceptions, but also for the reason that some investigators have vainly tried to find a refractory period in certain forms of living substance. _Langendorff_ and _Winterstein_,[135] for instance, have not succeeded in proving a refractory period for the spinal cord of the frog. _Langendorff_ stimulated the central sciatic stump with two stimuli in quick succession and used the contractions of the triceps as indicator of the response. He found that when the stimuli, if consisting in either single induction shocks or faradic shocks, followed each other even at intervals of .004 seconds the second stimulus was still operative, this being perceptible in an increase of the contraction or with greater intervals of time in a summation of two contractions. _Winterstein_ concludes from this that the development of a refractory period after a stimulation is not a general property of all nerve centers. If the experiments of _Langendorff_ failed to show the presence of a refractory period it is not for the reason that this does not take place in the centers of the spinal cord but rather results from the fact that the conditions for the investigation were not suited for its demonstration. In fact, _Fröhlich_[136] and especially _Vészi_[137] have incontestably proved the existence of relative refractory periods in the normal spinal cord.