The founder of the doctrine of the irritability of living substance is _Francis Glisson_ (1597–1677), member of the _Collegium Medicum_ in London and at the same time Professor in Cambridge. It is a fact also not altogether without interest, that _Glisson_ at the same time was in a certain sense a forerunner of those who interpreted nature from a physical standpoint. _Glisson_ as an anatomist and physiologist was an excellent observer and experimenter, but the most prominent trait of his character was his inclination to philosophic observation and analysis of nature. His “_Tractatus de natura substantiæ energetica_”[1] must, therefore, be considered as the chief work of his life. In this voluminous book _Glisson_ develops an entire system of natural philosophy, which in accord with the character of the philosophy of that time is unfortunately of an absolutely speculative nature and which had hardly emancipated itself from the scholasticism of the preceding period of thought. When the ideas of _Glisson_ are isolated from the wilderness of scholastic phraseology, the system is somewhat as follows. The basis of all existence, “_substance_,” has according to him two general properties, its “_fundamental subsistence_,” that is, the essence of its being, and its “_energetic subsistence_,” that is, the essence of its activity. To these are added the properties possessed in specific cases, that is, its “_additional subsistence_.” The energetic subsistence forms the basis of all life. Life is therefore present not only in organic nature, but in all nature which is characterized by the union of the general energetic subsistence with the special additional subsistence of an animal and vegetable nature. In other forms of life in nature the energetic subsistence is combined with other special forms of the additional subsistence. The universal essence of all life, that is the energetic subsistence, has only three fundamental faculties: the “_appetitiva_,” the “_perceptiva_” and the “_motiva_.” The _modus_ is the result of a “_perceptio_,” but the “_perceptio_” is not thinkable unless the object has the “_appetitus_” to receive the external influence. _Glisson’s_ doctrine of irritability is based on this conception, which he develops in a second work already begun before the “_Tractatus de natura substantiæ_,” but not finished until later and only published after his death. In this “_Tractatus de ventriculo et intestinis_,”[2] _Glisson_ dwells in detail on the physiological properties of animal structures and develops for the first time his conception of irritability in the chapter “_De irritabilitate fibrarum_.” The “irritability” manifests itself in the appearance of the alteration of movement, which is brought about by external influences on the animal structure, for: “_Motiva fibrarum facultas nisi irritabilis foret, vel, perpetuo quiesceret, vel perpetuo idem ageret._” The fundamental