of duration that the chemical alterations as a result of excitation are so increased as to be plainly perceptible with the aid of our chemical indicators. Unfortunately, we do not possess specific chemical indicators for every individual molecular constituent process of the cell and so cannot dispose with the help of indicators of the combined happenings in a greater quantity of living substance. It remains for us to obtain data concerning the cycle of excitation processes in the living substances by the aid of the combined employment of the most varied kinds of physical as well as chemical indicators. If we use the most varied types of living substance of widely differing properties, showing us the greatest variety of vital manifestations, we may hope by the use of comparative physiological methods, even though with difficulty, to separate more and more the essential details of the general processes of excitation. At present we are still at the very beginning of this task and vast fields of unexplored regions are yet before us. But it is the unknown which has a particular fascination, especially if we succeed from time to time in making new advances. If we suppose a living system in a state of metabolism of rest influenced by an instantaneously excitating stimulus, the entire course of excitation extends from the first alteration produced by the stimulation until the complete restitution of the metabolic equilibrium, and we will, therefore, differentiate individually the successive stages of this whole process. The very beginning of the chain of alterations produced by the excitating stimulus cannot be studied by any indicator. The changes must first reach a certain dimension by conduction from the point of stimulation before they influence even the most delicate indicators. The application of the stimulus is, therefore, followed at first by a measurable “_latent period_,” in which the living substance remains apparently at rest. This latent period has been particularly studied in muscle. After its discovery by _Helmholtz_[55] it was made the object of innumerable investigations and met with an interest which can only be explained by the exactness of the methods employed. Among others _Tigerstedt_[56] has made the most thorough study of the influence of various factors on the duration of the latent period. These experiments have established the fact that the duration of the latent period varies according to the intensity of the stimulus, temperature, loading or fatigue. This is apparent when it is understood that the amount of the alterations produced by the stimulus must ascend from the value zero to a certain height before the changes are perceptible, and that under various conditions this amount is, on the one hand, attained in different lengths of time and, on the other, must reach a varying amount before it is perceptible by means of the indicator.