CHAPTER XII 



STIMULATION AND TRANSMISSION OF EXCITATION 



IN PROTOPLASM 



Responsiveness to stimulation is a universal character- 

 istic of living matter. Typically the reaction to a 

 stimulus involves a performance of work (i.e., trans- 

 formation of energy) which has no definable proportion 

 to the work done by the stimulating agent upon the 

 living system. The stimulus usually acts locally, yet 

 the whole living system — cell, tissue, or even entire 

 organism — may be thrown into activity. Transmission 

 of physiological influence from the immediate site of 

 stimulation to other regions of the living system is 

 thus a constant feature of stimulation. Hence the 

 subject of the essential conditions determining this 

 transmission is one of fundamental biological interest; 

 evidently the living system can react as a whole, i.e., in 

 a unified or correlated manner, only in so far as the 

 physiological processes in any single region occur in 

 correlation with those in other regions. This trans- 

 missive property of protoplasm is the primary integrative 

 factor in organisms; its highest development has been 

 attained in the nervous system of higher animals.^ 



In general, local variations in protoplasmic activity, 

 implying variations in the rate or character of the 

 underlying chemical or metabolic processes, influence 

 other processes occurring at a distance from the active 



^ Cf. my review of the subject of protoplasmic transmission in 

 Physiological Reviews, II (1922), i. 



259 



