GENERAL CHARACTERS OF LIVING ORGANISMS 43 



facts illustrate the unitary character and control of the 

 metabolic processes underlying the various vital manifes- 

 tations; they show that growth and development are 

 controlled by the same conditions as the other forms of 

 protoplasmic activity. Hence stimulation is a concep- 

 tion which is applicable to growth processes in the same 

 sense as to muscular or nervous activity. 



Constructive metabolism thus varies with the general 

 physiological activity of the living system; and this 

 latter activity is determined largely by the external 

 agents which act upon or ''stimulate" the protoplasm. 

 The general property of ''irritability" thus implies not 

 only the ability of the protoplasmic system to carry 

 out definite reactions in response to stimuli but also 

 the ability to vary its constructive metabolism in 

 correlation with the rate or degree of the energy-yielding 

 or destructive processes. Restitution, compensatory 

 growth, recovery from injury, or fatigue and apparently 

 the normal recovery of the irritable state after stimulation 

 are different manifestations of this constructive process. 



GENERAL FEATURES OF STIMULATION PROCESSES 



In general, the term "irritability," as used in physiol- 

 ogy, designates the universal property of living matter 

 by which the chemical or other activities of the living 

 system change, in some specific way, in response to 

 changes in the surroundings. We say "change in some 

 specific way", i.e., in a manner distinctive of the living 

 system, in order to separate true cases of stimulation 

 from cases where the chemical or other processes occurring 

 in the protoplasm are changed as a direct consequence 

 of non- vital factors. For example, within the usual 



