[55] _Helmholtz_: “Messungen über den zeitlichen Verlauf der Zuckungen animalischer Muskeln and die Fortpflanzungsgeschwindigkeit der Reizung in den Nerven.” Archiv für Physiologie Jahrgang 1850. [56] _Robert Tigerstedt_: “Untersuchungen über die Latenzdauer der Muskelzuckung in ihrer Abhängigkeit von verschiedenen Variablen.” Arch. f. Physiologie Jahrgang 1885 Suppl. The facts concerning the whole latent period and its dependence on various factors would be incomprehensible if it were assumed that no alterations whatever take place during the latent period although the stimulus is already operative. In reality, the alterations following a stimulus occur with imperceptible rapidity in the form of a molecular interchange, and the latent period is simply an expression of the fact that the primary alterations, being limited in nature, are not registered by our indicators. The question first arises, In what do these first imperceptible alterations consist? _Nernst_[57] has evolved the theory for electric stimulus, that the primary effect produced by the electric current is an alteration in the ion concentration on the surface of the living substance. In fact, we know that the surfaces of all protoplasm possess the property of semi-permeable membranes and that changes in the concentration of ions invariably occur when an electric current flows through two electrolytes separated by a semi-permeable membrane, in which the anions and cations have a different rapidity of movement. It is apparent, therefore, that such an alteration in the ion concentration must be followed by further chemical processes in the living substance. According to the theory of _Nernst_ the first impetus for all further alterations, which the electrical stimulus brings about in the metabolism of rest, is the alteration in the concentration of the ions on both sides of the semi-permeable membrane, which represents the surface of the protoplasm. In view of the present findings of physical chemistry, objections can hardly be made to this theory of _Nernst’s_. It is a question, however, in how far this theory, especially established for the _electric_ stimuli, can be applied to other forms of stimuli and their action. It cannot be denied that the degree of dissociation of an electrolyte can be altered by very different factors, such as heat, light, chemical processes, etc., and in that the surfaces of the protoplasm, acting as semi-permeable membranes, bring about a selective action on the passage of the ions, there arises the opportunity for the development of difference of electrical potential on both sides, and for further chemical alterations in the protoplasm. These observations, however, require further experimental investigations in many fields, before we are justified in extending the _Nernst_ theory of the manner of action of the electric stimuli to a general explanation of the primary alterations produced by all stimuli in the living substance. For the