ORGANIC RESPONSE 247 



changed quantitatively, i.e. increased or decreased, 

 whence the reactions are sometimes termed excita- 

 tions and depressions. The chemical stimulus of 

 substances known as narcotics produces a depression 

 of protoplasm. Increase of heat produces excita- 

 tion, decrease (cold), depression. For each kind of 

 protoplasm there are minimal and maximal limits of 

 stimulation within which it displays its characteris- 

 tic vital phenomena, but the nature of the reaction 

 to any stimulus whatever is of course dependent upon 

 the peculiar characteristics of protoplasm itself ; in 

 other words, upon its organic make-up. In propor- 

 tion as protoplasm becomes specialized the nature 

 of its response becomes more and more restricted. 

 Any stimulus applied to a muscle, e.g., causes the 

 same sort of a response (contraction) whether the 

 excitant be electrical, thermal, mechanical, or chemi- 

 cal. This specificity of response is sometimes known 

 as the law of specific energy. 



Electric Response. In the majority of cases we 

 recognize the existence of a stimulus and a response 

 by a change of form, but excitation and response 

 may be present without being evident to our senses. 

 A nerve when stimulated shows no change in itself 

 even though it transfer its stimulation to the muscle 

 with which it is connected. Nevertheless if we lay 

 across a nerve two electrodes, connected through a 

 galvanometer, and stimulate the nerve by pinching 

 it, we will see by the deflection of the needle of the 

 galvanometer that an electrical change has taken 



