44 PHYSIOLOGY CHAP. 



and environment is intimately connected with metabolism as 

 exchange of energy. Each living organism contains within itself 

 at any given moment of its life a sum of potential energy, drawn 

 from the sun's rays, and from the food-stuffs which it has ac- 

 cumulated or assimilated ; and this energy is always ready to 

 discharge itself, or explode by transformation into kinetic energy, 

 in consequence either of internal impulses or of external stimuli. 

 The most striking form assumed by the energy developed in a 

 living organism is the movement of masses, the power of surmount- 

 ing resistance, i.e. of doing mechanical work. When these move- 

 ments or changes of form or position in space depend upon internal 

 stimuli they appear to be spontaneous or automatic, and are the 

 most common and obvious objective sign that the organism that 

 accomplishes them is living. When they are provoked by external 

 stimuli they appear as reflex movements, i.e. as the effects of 

 internal reactions to external stimuli ; in that case there is a 

 striking disproportion between action and reaction, although this 

 is not a distinctive sign of life, since the same may be observed in 

 many chemical combinations the so-called explosives. What 

 does, however, differentiate the latter from living substances, is 

 that the chemical activity of explosives exhausts itself in the 

 explosion, while the organism becomes fatigued with work, and 

 recuperates in repose, i.e. at each reaction it only discharges part 

 of its energy, and during the functional pauses it recovers by the 

 anabolic process the quantity of potential energy that has been 

 consumed. 



This peculiar capacity for developing, spending,, and reaccum- 

 ulating energy, which characterises living beings, has received 

 the name of Excitability, and is distinguished as reflex or auto- 

 matic, according as the reactions or excitations are provoked by 

 internal tendencies or impulses, or by external stimuli, or excita- 

 tions extrinsic to the organism. 



That there must be a marked analogy between the internal 

 conditions of automatic, and those of reflex excitability, appears 

 from the fact that it is often very difficult to differentiate objec- 

 tively between automatic movements and reflexes; on the other 

 hand, many movements originally automatic become reflex by 

 a simple morphological evolution of the elementary organism 

 that produces them, while many originally reflex movements 

 become automatic by long exercise and habit. 



These phenomena of excitability, which can be observed under 

 various forms in all living organisms, are intimately connected 

 with another group of phenomena, that can be directly observed 

 upon ourselves alone, since they are accessible only to immediate 

 internal observation or introspection. These last are the psychical 

 phenomena, which as a whole constitute the content of con- 

 sciousness. 



