I will call your attention later to the fact that these sub-threshold stimuli play a very important rôle under certain conditions in the activities of the central nervous system. It only depends upon the sensitivity of our special senses, or the indicators used for this purpose, as to whether the alterations can be observed or not. The conception of the threshold of stimulation, therefore, has meaning only when used in relation to a certain indicator. The threshold of the same living system may be different for different indicators. When we use the term threshold we must necessarily know the indicator employed in its determination. The threshold stimulus produces only barely perceptible effects. The amount of response in most living substances increases with the intensity to a certain limit. If this limit is reached, that is, if the response is maximal, the stimulus of the weakest strength necessary to produce this result is termed the _maximal stimulus_, whereas all intensities lying between the threshold and the maximal stimulus are termed _submaximal stimuli_. If the intensity of the stimulus is increased _above_ that of the maximal, the response, as in the case of the muscle, does not increase, and therefore one could say that all intensities above the maximal could also be called maximal stimuli. In realty, however, the response to stimuli of different intensities is never equal, even though it may appear so, when measured by an indicator, as for instance, the height of the maximal muscle contractions. This is clearly shown, for example, when the electrical stimulus is increased far beyond that intensity which is necessary to produce maximal effect. Injury is thereby produced, which is manifested, for instance, in the muscle contraction by the nature of its course and also by its height. One is, therefore, justified in a certain sense in calling the intensities of the stimulus, which are above the value which barely produces maximal contraction, “_supermaximal stimuli_,” notwithstanding this is logically far from being a happy expression. The term “maximal stimulus,” then, is limited to the intensity of the stimulus which just produces a maximal effect. I wish to point out this distinction between maximal and supermaximal stimulus, as there is often a lack of clearness in the use of these terms.