522 TRANSFORMATION OF ENERGY 



influence of the environment, and this influence is twofold ; in the first place, 

 it provides the general {formal) conditions, in whose absence no movement in 

 response to stimulus — nor indeed any other vital phenomenon — can take place, 

 and it also provides specific stimuli. Both these effects of the environment require 

 further elucidation, and we may start with a consideration of specific stimuli. 



We have already described these movements in response to stimuli as 

 released movements (Pfeffer. 1893 ; Physiol. I, 9 and II, 80) ; it should be 

 added that the factors which act as stimuli are merely the inducing causes of the 

 movements which the organism carries out by its own inherent energy ; the 

 stimulus itself never provides the energy actually required for carrying out the 

 movement. It follows that no definite relation exists between the energy 

 of the stimulus and that manifested in the response. The response is always 

 accompanied by an expenditure of energy, but the stimulus may be effected just 

 as readily by a withdrawal as by an application of energy. An example of the 

 former is seen in the stimulus induced by a reduction in temperature (compare 

 p. 502). It may be said that the stimulus releases the reaction, it causes or 

 induces it, and hence we speak of induced movements ; we further ascribe to the 

 plant the power of perceiving the stimulus or of being sensitive to it, hence the 

 term 'receptive movements '. The stimuli we have previously dealt with have 

 been external stimuli, such as light, heat, electricity, gravity, chemical and 

 mechanical effects of certain substances, &c. ; but, just as in the differentiation 

 of the plant so in its movements there are internal stimuli concerned, some of 

 which we have to study in this lecture. 



Since the essential characteristic of a stimulus lies in the fact that it acts 

 as a releasing agent, it follows that such stimuli are not confined to organisms ; 

 indeed we constantly make use of such releasing agents in machines of various 

 kinds, and since the causal connexion is incomparably more apparent in them 

 than in organisms, owing to the simplicity of the conditions, we can explain the 

 principle of a releasing agent in a machine far more easily than we can in an 

 organism (Pfeffer, 1893). Let us take the case of an electric bell. The apparatus 

 consists of a bell, the electric battery, and the conducting wire. The bell is the 

 mechanism which is stimulated by an electric current emanating from the battery. 

 Ordinarily, however, the conductor which runs from the source of electricity 

 to the bell is interrupted and a sound is produced only when the current is 

 ' closed '. This 'closing ' or joining of the wires is the point of interest at present, 

 since it is the closing that ' releases ' the bell's capacity for making a noise. In 

 order to make and break the electric circuit a key is used, and the necessary 

 metallic connexion between the terminals is made by means of a gentle pressure 

 on a metal plug. It may be seen at once that the amount of pressure employed 

 stands in no relation whatever to the loudness of the resulting clang. According 

 to the way in which the key is constructed it may require the slightest touch 

 of the finger or the whole of a man's strength to close the circuit, but, so long as 

 the pressure is sufficiently great to effect closure, the bell responds in the same 

 way, provided the electric current passing along the wire remains unaltered. 

 When we remove the pressure from the button the key returns to its rest position, 

 the current is interrupted, and the bell ceases to ring. 



There would be no great difficulty in manufacturing other kinds of * keys ' 

 by which the current could be closed by magnetic energy, electricity, heat, or 

 light, instead of by mechanical means. It is quite unnecessary for us to spend 

 time in describing how such an apparatus could be made, it will be sufficient 

 for our purpose to note that a new type of key will be required for each type of 

 releasing agent. No matter what the structure of the key, the bell reacts only 

 to a single releasing agent ; the mechanism responds to this influence, or, as we 

 might say, is sensitive to it. It is also obvious that the application of pressure 

 to any part of the system other than the key, i. e. the application of the releasing 

 agent to any other place than the sensitive apparatus, must be quite ineffective. 



