264 PROTOPLASMIC ACTION AND NERVOUS ACTION 



instances would scarcely be classed as responses to 

 stimulation. In fact many opportunities for verbal 

 mystification arise in attempting to ''define" the concept 

 ^'stimulation." In order to limit the following discus- 

 sion, we shall regard as a "stimulus" any influence 

 acting from without upon the living system which changes 

 the rate or the character of the normal vital activities; 

 the resulting change of physiological activity is the 

 "response." Even with this simplified conception, the 

 range of phenomena is still too great to be readily 

 included under any strictly drawn definition; but such 

 a definition need not be insisted upon, provided the 

 general nature of the relations between organism and 

 environment is clearly understood. Variation of vital 

 activity, within the physiological range, occurring as a 

 constant correlative or sequence of environmental 

 change of some kind, is the essential phenomenon 

 whose conditions we are considering. 



In multicellular organisms, "internal" and "external" 

 (proprioceptive and exteroceptive) stimuli are often 

 distinguished,^ since in many cases the environment 

 which furnishes the normal stimuli for an irritable cell 

 or cell-system may be not the external world but some 

 other part of the same organism. Responses of special 

 organs or organ-systems to stimuli originating else- 

 where within the same organism form, in fact, a 

 regular part of many normal physiological cycles in 

 higher animals; thus the pancreas is stimulated by 

 secretin in the blood stream, and the respiratory 

 center by increased H-ion concentration of the blood; 

 the central nervous system is continually adjusting its 



* Cf. Sherrington, Integrative Action of the Nervous System. 



