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Adrian draws attention to the fact that the physical sciences have made the term 'energy' so precise and full of meaning that it is difficult to ignore this meaning, and there is therefore danger of investing the term with the same meaning when we apply it to the data of the biological sciences—a confusion which, it is suggested, is to be found for instance 'in semi-popular explanations of the theories of Freud'. In speaking of nervous or mental energy in the biological or psychological sense we must beware of assuming that such energy necessarily obeys the laws of physics (i.e. the conservation of energy). It is true that this objection does not apply to the use of the term 'nervous energy' to mean a special manifestation of physical energy peculiar to the living nerve fibre but transferable into other forms of energy in accordance with the laws of thermo-dynamics. But the evidence at present available tends to show that the energy degraded in the passage of the nervous impulse is in essential respects quite similar to the manifestations of energy in non-living systems. Thus, to sum up, the concept of a special nervous energy in this latter sense is unnecessary, and in any other sense is dangerous.
Head is concerned, not with the isolated nerve fibre, or the isolated reflex, but with the nervous system as a whole. He considers that the efficiency of the nervous system can profitably be considered as a function of the state of 'vigilance' of this system. The greater the 'vigilance' (either of the system as a whole or of the part most directly concerned), the smaller the stimulus required to produce a response, and the more efficient—both quantitatively and qualitatively—the response when produced. This doctrine of 'vigilance' is considered in some detail with regard to various levels of physiological and psychological functions.