[65] _Gustav Mann_: “Histological changes induced in sympathetic motor and sensory nerve cells by functional activity.” In Journ. of Anat. and Physiol. 1894. Further: _Gordon Holmes_: “On morphological changes in exhausted ganglion cells.” Zeitschrift f. allgem. Physiol. Bd. II, 1903. [66] _Wallengren_: “Inanitionserscheinungen der Zelle.” Zeitschrift f. allgem. Physiol. Bd. I, 1902. [67] _W. Pfeffer_: “Ueber die regulatorische Bildung von Diastase.” In der math. phys. Klasse d. Königl. Sächs Ges. d. Wiss. zu Leipzig 1896. [68] _De Bary_: “Sur la fermentation de la cellulose.” In Bull. de la Soc. bot. de France 1879. Entirely different is the question if the cell also possesses a reserve store of oxygen. In this respect views have widely differed, and even today no conformity of opinions has been arrived at. The fact that many purely aërobic organisms and tissues can exist under complete exclusion of oxygen for a longer or shorter period, retaining their excitability and producing carbon dioxide, has for a long time led a great number of investigators, such as _Liebig_, _Matteucci_, _Engelmann_, _Pettenkofer_ and _Voit_, _Claude Bernard_, _Verworn_, _H. v. Baeyer_ and others, to the supposition that a reserve store of oxygen must exist in the living substance which maintains its excitability for a time. More recent information, however, of the transition of the oxydative to the anoxydative disintegration under a deficiency of oxygen, as can be observed in plants and certain invertebrate animals, indicates that here also there is the possibility of another explanation of these facts. Various attempts have been made to solve the problem if reserve oxygen is present in the cell or not. The experiments of _Rosenthal_,[69] carried out with his respiration calorimeter, seemed to point directly to an oxygen reserve in the organism of the mammal. He observed that during respiration in an atmosphere rich in oxygen the respiratory quotient (CO_{2} : O_{2}) became lower than in ordinary air, that is, that oxygen, and that indeed in considerable quantity, must be retained in the organism. Nevertheless _Falloise_[70] found that when rabbits, which had been kept in an atmosphere containing 80 per cent of oxygen, were asphyxiated, the time necessary to produce death was no longer than in animals which had been kept previously in ordinary air. The correctness of the observations of _Rosenthal_ have been disputed by _Durig_.[71] _Winterstein_[72] also, employing the microrespiration methods of _Thunberg_ upon the spinal cord of the frog, believed that he had found proof that an oxygen reserve cannot take place. He reasoned thus: If the cells of the spinal cord contain reserve oxygen, which is used up when pure nitrogen only is breathed, then it necessarily follows that after reintroduction of oxygen, following asphyxiation, a definite quantity must be stored up again as reserve. In consequence, the