(3) Energy of muscle and nerve. This refers to the principle of the 

 storage and discharge of energy, and the biological theory that 

 functional activity of a specialized tissue depends primarily upon 

 chemical changes in its individual cells. The fundamental idea is that 

 in the resting state the cell elaborates highly complex compounds 

 and that these break down to yield the energy by which the cell does 

 its work; discharge and restoration of energy is common to both 

 nervous and muscular elements. Hughlings Jackson characterizes 

 the animal organism as "an apparatus for the storage and expendi- 

 ture of nerve force." These principles are of essential importance in 

 the study of mental disorders. Inasmuch as functional efficiency 

 must be taken as a measure of the available energy, it should be 

 expected that exhausting influences would reduce functional power. 

 Such reductions characterize all forms of the functional psychoses, 

 and the variations of their symptoms are consistent with this 

 principle. 



(4) Physiological use and fatigue, waste and repair. The law of 

 use includes the wholesome effects of those just cited; normal use 

 develops functional activity and strengthens power, while disuse 

 weakens function. Overuse begets fatigue, and normal fatigue pre- 

 sents mental as well as physical effects. Physiological fatigue may be 

 continued beyond the point of regular recovery by rest and nutri- 

 tion ; it then becomes the pathological fatigue of nervous exhaustion 

 or neurasthenia with the characteristic symptom-groups. A func- 

 tional conception of the significance of these groups of mental and 

 physical symptoms should stimulate not only such a precise obser- 

 vation of them as is needed to constitute " disease-forms " and 

 mature types, but should lead to their being analyzed and traced to 

 their functional sources in the whole organism in accordance with the 

 principles of general pathology. This method reveals the genesis in 

 physical states of some of the most characteristic mental manifesta- 

 tions. Beginning with the fundamental attribute of irritability, for 

 example, wide variations occur within normal limits, but more 

 striking and significant changes appear in all forms of pathological 

 fatigue, and the functional psychoses; the irritable weakness and 

 languor of neurasthenia, and the psycho-motor excitations, retarda- 

 tions, and "confusions" of melancholia and mania are examples. 

 The study of these alterations of irritability involves the whole pro- 

 blem of reflex-action and the mechanism of responses to stimulation 

 of both mental and physical functions. It is to be recognized also 

 that all of these reactions contribute to the sensory returns from 

 the whole organism, from the viscera, muscles, and even the spe- 

 cial senses including the special dermal sensations, to the central 

 nervous system, constituting the kinesthetic and organic sensations. 

 In mental physiology a functional conception of these reactions 



