GENERAL CHARACTERS OF LIVING ORGANISMS 45 



stimulus may produce entirely different effects upon 

 different irritable systems, or upon the same system at 

 different times. For example, the same intensity of 

 light will repel one group of animals, and attract another; 

 mechanical treatment may arouse increased activity in 

 one motor organ (a muscle) and inhibit it in another 

 (the swimming plate of a ctenophore). The case just 

 cited is interesting as illustrating another general feature 

 in the behavior of irritable systems; the swimming plates 

 of Mnemiopsis or Eucharis beat rhythmically with 

 considerable regularity, but instantly cease movement 

 when mechanically stimulated in the presence of sufficient 

 Ca salts; e.g., in sea water or artificial media containing 

 calcium; but in similar media containing no calcium, 

 mechanical treatment entirely fails .to inhibit the move- 

 ment, and on the contrary accelerates it.^ This instance 

 shows that the same external change of condition may 

 produce different effects in the same tissue according to 

 its physiological state at the time; under one condition 

 there is an inhibitory, under another an acceleratory 

 response. Electrical stimulation of the nerve supplying 

 a voluntary muscle causes the latter to contract; but 

 the same stimulus applied to the cardiac branch of the 

 vagus nerve inhibits contraction. 



Such examples illustrate the distinction between the 

 stimulating effect of an agent or change of condition 

 upon an irritable living system, and the direct effect 

 which it produces by its purely physical or chemical 

 action upon the system. Superposed upon and sequent 

 to the direct physico-chemical effect is the special or 

 physiological effect, the nature of which depends on the 



^ R. S. Lillie, American Journal of Physiology, XXI (1908), 200. 



