Fear is a factor in many injuries and operations. The phenomena of fear probably are exhibited only by animals whose natural defense is nerve-muscular. The skunk, the porcupine, the turtle, have little or no fear. Fear is born of the innumerable injuries which have been inflicted in the course of evolution. Fear, like trauma, may cause physiologic exhaustion of and morphologic changes in the brain-cells. The representation of injury, which is fear, being elicited by phylogenetic association, may be prevented by the exclusion of the noci-association or by the administration of drugs like morphin and scopolamin, which so impair the associational function of the brain-cells that immunity to fear is established. Animals whose natural defense is in muscular exertion, among which is man, may have their dischargeable nervous energy exhausted by fear alone, or by trauma alone, but most effectively by the combination of both. What is the mechanism of this discharge of energy? It is the adequate stimulation of the nociceptors and the physiologic response for the purpose of self-preservation. According to Sherrington, the nervous system responds in action as a whole and to but one stimulus at a time. The integration of the individual as a whole occurs not alone in injury and fear, but also, though not so markedly, as a result of other phylogenetic associations, such as those of the chase and procreation. When adequate stimuli are repeated with such rapidity that the new stimulus is received before the effect of the previous one has worn off, a higher maximum effect is produced than is possible under a single stimulus, however powerful. Sexual receptors are implanted in the body by natural selection, and the adequate stimuli excite the nerve-muscular reactions of conjugation in a manner analogous to the action of the adequate stimuli of the nociceptors. The specific response of either the sexual receptors or the nociceptors is at the expense of the total amount of nervous energy available at the moment. Likewise in daily labor, which, in the language of evolution, is the chase, nervous energy is expended. Under the dominance of fear or injury, however, the integration is most nearly absolute and probably every expenditure of nervous energy which is not required for efforts toward self-preservation is arrested; hence fear and injury drain the cup of energy to the dregs. This is the potential difference between fear and desire, between injury and conjugation.