I have treated the views of _Glisson_ somewhat in detail for on the one hand this seemed to me to be only due to the founder of the doctrine of irritability, and on the other we have here for the first time, although in somewhat vague and little worked out form, the discovery of a general property of all living substance, and its fundamental importance for the life of the organisms. One might, therefore, in a _certain_ sense, date from _Glisson_ the beginning of general physiology, and all the more so, because _Glisson_ from the very first connected the irritability of the living substance through its possessing universal energy with the phenomena in nature generally, just as we do today two hundred years after, on the basis of the modern teachings of energy. It might appear strange that a teaching of such fundamental importance as that of _Glisson’s_ theory of irritability was not at once accepted on all sides and further developed. There were two reasons, however, which prevented this. Firstly, _Glisson_ did not devote himself to his post of teacher at the University of Cambridge with any particular zeal and so consequently did not establish a school of his own, to further work out and develop his ideas. Secondly, his doctrines were so speculative and difficult to understand, his differentiations and definitions so artificial and labored, that it required the greatest effort to penetrate to his fundamental conceptions and so it happened that _Glisson’s_ theory of irritability received attention only at a comparatively late date. Even then, of his speculative theories hardly more than the name “doctrine of irritability” was adopted. Since the middle of the eighteenth century this name, however, was destined to lead to excited controversies.