I n the absence of oxygen a nerve gradually loses its ability to conduct; when the gas is made available conduction promptly returns. The lack of oxygen might condition failure of conduction via several mechanisms : 1) decrease in energyyielding reactions with a consequent breakdown in the functionable machinery, which depends on a continuous energy How for its integrity; 2) diminution of a particular needed substance not reformed in the absence of oxygen or, the converse, accumulation of a harmful one not removed in its absence; 3) a less specific alteration in the milieu, induced by oxygen lack, and leading secondarily to interference with the energetic or chemical turnover required for maintained activability. Clearly such itemization of a unified physicochemical system introduces an element of artificiality, yet the gain in precision seems to justify it. The slow rather than immediate loss of function has been attributed to the presence in the tissue of partially oxidized substances that may serve as hydrogen acceptors until reduced (oxidizing reserve), and to the occurrence of other, non-oxidative, energy yielding reactions (energy reserve) (Gerard, '32 a).