THE NATURE OF STIMULATION 37 



muscle cells as a stimulus. Although the explosion of the im- 

 pulse is an alteration within the body, nevertheless, as far as the 

 muscle is concerned, it may be looked upon as an external vital 

 condition, therefore as a stimulus. As the conception of stimulus 

 involves the relation to a given state, it likewise involves at the 

 same time the relation to a given living system, upon which it acts 

 from the exterior. 



What is the value then of all this theoretical discussion? 



In presenting the conception of stimulation from a conditional 

 standpoint, I desired to show what difficulties stand in the way 

 of a theoretical isolation of a fundamental conception in the field 

 of physiology, which indeed is used in our practical research 

 work at every step. "Natura non facit saltus." I wished to 

 demonstrate that the sharp separation of the conception of stimu- 

 lation, like all artificial divisions which we make in nature, must 

 always contain an arbitrary note, as in reality isolated systems 

 do not exist in the world. I wished to show that, for this reason, 

 the conception of vital system, the conception of life, the con- 

 ception of vital conditions are not sharply defined. I wished 

 likewise to show that as a necessary consequence of this fact 

 a sharp separation of the conception of stimulation, which can 

 only be made in relation to that of vital conditions, cannot be 

 maintained theoretically. I wished to show further that there 

 is no sharp line of division between inner and outer vital con- 

 ditions, and that we cannot, therefore, make a strictly theoretical 

 distinction between the conception of stimulation and that of 

 the processes of development. I wished to show that, for these 

 reasons, we must not expect from the conception of stimulation, 

 as we understand it, anything beyond its possibilities. But finally 

 I wished also to show that, whilst fully conscious of and with 

 due consideration of all these difficulties, it is possible to work 

 out a definition of stimulation which is of great practical work- 

 ing value. The definition in short is : "Stimulus is every altera- 

 tion in the external vital conditions." 



This definition gives to the conception of stimulation its most 

 complete, that is to say, its generally applicable and simplest 

 form. The great importance from a methodical standpoint of 



