STIMULATION OF LIVING ORGANISMS 589 



why the same tissue may respond in the same way to so many different 

 stimulating agencies. Any agency that alters the surface-film to a de- 

 gree and at a rate sufficient to cause a critical change in its electrical 

 polarization will stimulate. The membrane may be directly altered by 

 mechanical agencies, or by heat or the direct action of chemical sub- 

 stances ; or it may contain photosensitive substances and hence be sensi- 

 tive to light, or special chemical substances and show a specific chemical 

 sensitivity. Whatever alters it in such a way as to change, even momen- 

 tarily and locally, its permeability and electrical polarization to the criti- 

 cal degree may thus stimulate, i. e., may originate a depolarization which 

 spreads and affects the entire cell. The response which then follows is 

 independent of the nature of the stimulating agency and is determined 

 by special peculiarities of the irritable tissue itself. 



The processes which take place in the interior of the stimulated cell 

 are too various and complicated to be considered here. Their nature 

 depends entirely on the specific peculiarities of the cell, and any general 

 characterization is impossible. Usually there is an increase of oxidations 

 and hence of heat-production — in addition to the special physiological 

 manifestation which is evoked — but this is not always the case ; thus in 

 nerve, although there is increased loss of carbon dioxide, the heat pro- 

 duced during activity is almost inappreciable; and in other cases there 

 may be a decrease or even complete cessation of all outward activities, 

 e. g., in structures that give an inhibitory response to stimulation ; such 

 an instance is seen in the swimming plates of ctenophores which stop 

 movement instantly on slight mechanical stimulation. Facts like these 

 again illustrate the extreme diversity which the entire sequence of events 

 forming the response may show in different irritable tissues, in spite of 

 the essential uniformity of the first stage of the process. This uniform- 

 ity is the most remarkable feature of physiological stimulation. Nature 

 has apparently found in the variations of permeability and of electrical 

 polarization which external changes may cause in the protoplasmic sur- 

 face-films the most effective and reliable means of which the internal 

 processes of the protoplasmic system can be made to vary in response to 

 variations in the environment; and in the course of evolution this 

 mechanism has acquired a degree of perfection that still largely baffles 

 physiological analysis. 



