of dry substances be regarded as consisting entirely of molecules capable of decomposition. For in this is also included the organic reserve material of the axis cylinder protoplasm, which is doubtless of very considerable amount. Further, the salts and products of disintegration, for which the estimate for the sum total would probably not be too low if we assume the amount to be equal to that of the group specially concerned in the process of excitation. As, however, a constant metabolism of rest takes place, these last molecules or atom groups are certainly not at the moment of entrance of the stimulus in their entirety in a condition capable of decomposition. It is quite certain, therefore, that we are still overestimating the amount of the molecules capable of disintegration, if we put them down as 5 per cent. of the entire nerve substance. If we now suppose that this 5 per cent. of irritable molecules are broken down as a result of stimulation, 95 per cent. of nonirritable substance, separating these irritable molecules, must become heated to such a degree by the disintegration of the latter that the amount of heat suffices to bring about decomposition of the nearest surrounding molecules or atom groups, for otherwise conduction of disintegration could not take place in this manner. This condition presents a serious difficulty for the assumption that heat is the form of energy responsible for the conduction of disintegration. It is true that we cannot reject this view at once as being completely incorrect, as the possibility of conduction does not depend upon the absolute amount of heat which reaches the next molecule capable of decomposition, but upon the relative amount of heat in regard to the degree of lability of the irritable molecules, of which we cannot even approximately make an estimate. However, by a comparison with other highly explosive substances, such as iodide of nitrogen, we find that a slight trace of water applied to the iodide of nitrogen suffices to prevent the extension of the disintegration process, and with this the explosion of the whole mass. Nor does the view of _Pflüger_ remove this difficulty, which assumes that the atom groups capable of breaking down are joined together by a chemical linking of atoms to long fiber-shaped giant molecules through the whole nerve fiber, for this assumption of a firm structure can hardly be reconciled with the principles concerned of metabolism. [113] Compare _Pflüger_: “Ueber die physiologische Verbrennung in den lebendigen Organismen.” In Pflügers Archiv. Bd. 10, 1875. Further: _L. Hermann_: “Handbuch der Physiologie, Bd. II, Allgemeine Nervenphysiologie,” 1879.