Finally, there is another factor to be considered, namely, that the latent period of the second stimulus is more and more prolonged as the second stimulus approaches more closely to the absolute refractory period of the first. In the above schemes the latent period was not taken into consideration because practically for all the intervals of stimulation considered at that time it could be assumed to be the same. When, however, a decrease of the intervals between the individual stimuli takes place, the prolongation of the latent period can then not be overlooked, as it leads to a retardation of response. (Figures 29, 30.) This fact was shown in the classic investigations of _Marey_[177] upon the refractory period of the heart, and more recently has been the subject of study by _Samojloff_,[178] _Keith Lucas_[179] and _Gotch_[180] in the muscle and nerve. These, then, are the essential factors which bring about interference, and although there are special details which deserve more close analysis, nevertheless, we are in a position to attribute to them the origins of summation and inhibitory processes, which occur in all living systems, especially the nervous system. [177] _Marey_: “Des excitations artificielles du cœur.” Trav. du lab. de M. _Marey_ II, 1875. The same: “Des mouvements que produit le cœur lorsqu’il est soumis à des excitations artificielles.” Compt. rend. de l’acad. des sciences T. LXXXVII, 1876. [178] _Samojloff_: “Actionsströme bei summierten Muskelzuckungen.” Arch. f. Physiologie Suppl. 1908. The same: “Über die Actionsstromkurve des quergestreiften Muskels bei zwei rasch aufeinanderfolgenden Reizen.” Zentralblatt f. Physiol. 1910. [179] _Keith Lucas_: “On the refractory period of muscle and nerve.” Journ. of Physiology, XXXIX, 1909–10. The same: “On the recovery of muscle and nerve after the passage of a propagated disturbance.” _Ibid._ XXXXI, 1910–11. [180] _Gotch_: “The delay of the electrical response of nerve to a second stimulus.” Journ. of Physiology, XXXX, 1910. For the analysis of summation and the inhibitory processes which occur in the physiologically active organisms or which are experimentally produced, a very important point should be observed, that is, the fact that the stimuli which bring about these phenomena are practically always a _series_ of _single_ stimuli. The nerve impulses, for example, consist of a shorter or a longer series of single discharges which follow each other in rapid rhythmic sequence. Here, then, we have the conditions necessary for the production of interference effects when these single stimuli follow each other with sufficient frequency and also when there is the combined action of _two_ series. [Illustration: Fig. 53.