METABOLISM. IRRITABILITY 3 



energy is created or destroyed, and that energy can only 

 change from one form to another. By means of dissimila- 

 tion the stored-up chemical potential energy of the organic 

 substance is changed to kinetic energy, mainly to heat and 

 mechanical work, and, in smaller amounts, to electric force 

 (e.g. in the electric fishes) and light (e.g. in fire-flies). The 

 kinetic energy set free by dissimilation enables the living 

 body to perform its functions. 



By assimilation kinetic energy is transformed into chem- 

 ical potential energy and is stored up in the organic sub- 

 stances. This energy is derived ultimately from the 

 sunlight. Only in the presence of sunlight are plants 

 containing chlorophyll able to assimilate. For assimilation 

 not all the light-rays are suitable, only the red and yellow, 

 not the green, blue, and violet. All energy which enables 

 the living being to perform its functions is, therefore, sun- 

 light changed to chemical potential energy. 



The changes of matter and energy in the living being can 

 be influenced by outside chemical and physical actions. Of 

 special importance are the influences by which the dissimila- 

 tion is increased. Such influences are called stimulating 

 agents or stimuli. The increased dissimilation brought 

 about by the stimulus is called stimulation, and the 

 resulting activity is the external expression of the stimula- 

 tion. The power of the living substance to respond to a 

 stimulus is called irritability. 



For example: A muscle stimulated by an electric current 

 passes from the condition of rest into the condition of activity; by 

 it the processes of combustion are greatly increased, the muscles 

 shortened, and work performed. 



The stimulus does not convey to the stimulated object 

 the energy set free upon stimulation, but only calls forth the 

 transformation of the already present chemical potential 

 energy into kinetic energy, just as the fuse of the gun calls 

 forth the explosion of the powder. Hence the effect of the 

 stimulation is not proportional to the stimulus. 



A stimulation produced at a given part of an irritable 



