In the functional disintegration of living substance, the last type is realized. Living substance contains loose complex combinations, and we know that functional disintegration is accompanied by the consumption of these organic combinations. In the functional disintegration of muscle substance the nitrogen-free groups are concerned, and we must, consequently, first consider the carbohydrates. However, without further study we should not generalize from that which is true in the case of muscle. There are other forms of living substances which contain different combinations, which disintegrate as a result of the contact of a stimulus and yield carbon dioxide. A clue as to which combinations in individual cases undergo disintegration as a result of excitating stimulation, is furnished by the metabolism of rest in the particular substance. Plants and micro-organisms have been investigated more thoroughly in this connection than animals. Plant physiology has demonstrated that the material employed for the CO_{2} formation and with it the production of energy is carbohydrate, but that, on the other hand, various plant organisms and protistæ also use a quantity of other substances, such as fats and protein, indeed even such comparatively simple organic combinations as alcohol, formic acid and methane. It may be accepted that in all these various instances of excitation of the functional metabolism as a result of stimulation, the specific respiratory material of the substance concerned is used in greater amount in the decomposition and likewise invariably yields carbon dioxide. The point of most essential interest for the analysis of the excitation processes is, above all, the _mechanism_ of the organic combustion and the associated energy production. Here we may base our observations on the disintegration of carbohydrates, which is most extensive in the animal as well as in the vegetable kingdom. We may now ask how dextrose, for instance, disintegrates in the living system into carbon dioxide, for it is this, or a sugar of similar chemical nature, which is generally concerned. Plant physiology, which here, as in many other respects, is in advance of animal physiology, has indicated two ways by which this can be accomplished in the living substance. One is oxydative, the other, _an_oxydative disintegration.