STIMULI AND THEIR ACTIONS 349 



Photic stimuli comprise changes of light. 



Electriccd stimuli comprise electrical changes. 



The above classes include all forms of energy that come into 

 relation with the organism. It is observed that in this enumeration 

 magnetism is wanting. But it is now known with certainty that 

 magnetism exercises no effect whatever upon living substance, 

 and cannot properly be termed a stimulus. To it was ascribed 

 at one time a most far-reaching and remarkable influence over 

 the living organism ; this was when the physician Mesmer popu- 

 larised the so-called " animal magnetism," and when the possibility 

 of magnetising human beings, animals and plants, by means of- 

 magnets was believed in. But later research, and especially the 

 discoveries of the Scotch physician, James Braid, showed that 

 the phenomena that were observed in those cases from which gross 

 deception was excluded were phenomena of hypnosis, and had 

 nothing whatever to do with magnetism ; in their production a 

 piece of glass, a polished button, a. gas-flame, or any other visible 

 object had the same significance as a magnet. In accordance with 

 the mysterious attraction that all mysticism is wont to exercise 

 over the human mind, there are found even at the present time, 

 not only among the visionary adherents of spiritualism, but even 

 among acute physicians, some who are convinced of the action of 

 strong magnets upon certain individuals, especially upon hys- 

 terical women. But from all observed cases sober investigation 

 has invariably torn away the veil of mystery, and has revealed 

 either fraud on the part of the " mediums " or self-deception on 

 the part of the observers. Careful experiments upon the influence 

 of magnets upon the living organism have always yielded negative 

 results. The recent, extended researches with very strong electro- 

 magnets by Peterson and Kannelly in America demonstrate the 

 utter ineffectiveness of magnetism upon living matter. 



Stimuli, therefore, comprise chemical, mechanical, thermal, 

 photic, and electrical changes in the environment of the organism, 

 and no others. 



2. The Intensity of the Stimulus 



In order to form a clearer, idea of the relation of stimuli to vital 

 conditions, we must turn our attention to the intensity of the former. 



Every external vital condition can be fulfilled in different 

 degrees : food, oxygen, etc., may be introduced in small or large 

 quantities ; the temperature may be low or high ; in brief, every 

 vital condition can vary gradually within very wide limits without 

 life thereby being endangered. Nevertheless, limits to most vital 

 conditions are known, both an upper and a lower limit, and these 

 are termed respectively maximum and minimum. Continual life 



