590 LECTURE XXXIV. 



Mmosa suddenly falls down limp, and its parts become folded together, or 

 when a similar effect is produced simply by sudden darkness. And matters are 

 similar in all phenomena of irritability, not only in plants but also in animals, 

 and in our own bodies. What qualitative similarity or quantitative proportionality 

 exists, for example, between the vibrations of the luminiferous ether and our 

 sensation of sight, the vibrations of the air and our sense of hearing ? What I 

 am here insisting on is still more evident in the innumerable reflex actions of the 

 human body. 



It will lead to clearness if we again try to render intelligible the meaning of 

 certain words, because by an inaccurate use of them at this stage the greatest 

 confusion may be introduced into the matter, against which we must guard our- 

 selves so much the more since the difficulties in the subject itself are sufficiently 

 great, and should not be rendered still greater by indefinite language. It will be 

 well to keep sharply apart from one another the three ideas, ' Stimulus,' ' Stimu- 

 lation,' and ' Irritability ' or ' irritable structure.' 



Stimulus is the name which I give to any alteration in the environment of the 

 irritable organs by means of which stimulation is caused. Experience shows that 

 changes in the intensity of the light, variations of temperature, alterations of elec- 

 trical conditions, instantaneous shocks, sudden pressure, etc. may act as stimuli. 

 Constant illumination, or constant temperature, etc., on the contrary, is not to be 

 regarded as a stimulus. However, it is conceivable that even with constant external 

 conditions of life, the internal states of the irritable organs undergo change, and that 

 thus their capacity for reacting towards unaltered external influences undergoes 

 differences, and this must then have the same effect as if the influence of the external 

 circumstances had been altered. The main point is, that any change whatever, 

 whether it originates from within or from without, is to be regarded as a stimulus ; 

 since if both the external and the internal conditions remain constant, no stimulus 

 whatever appears to occur. In most cases where phenomena of irritability are con- 

 cerned, the accuracy of these reflections is at once clear ; but there is a series of vital 

 phenomena very frequently occurring, which can scarcely be grouped elsewhere than 

 among the stimulations, but which nevertheless make their appearance with especial 

 clearness just where the external circumstances are constant. These are the move- 

 ments which I have previously designated autonomous or spontaneously periodic, 

 and to which we shall return later still more in detail. 



It has already been said that a conspicuous feature of the phenomena of irri- 

 tability lies in that they correspond with the stimuli neither qualitatively nor quantita- 

 tively, and it is in this very fact that the essential distinction between phenomena 

 of irritability and simply mechanical, physical, and chemical actions lies. It has 

 moreover already been shown above in what the explanation of this remarkable 

 circumstance essentially lies, viz. in the irritable structure of the organ. 



It will perhaps contribute to the intelligibility of this statement if we here 

 again figure to ourselves a few examples from inorganic nature. Something similar 

 to a phenomenon of irritability occurs, for example, in the case of a heated 

 steam-engine the valve of which is suddenly opened. Previously at rest, the 

 mere opening of the valve puts the engine in motion and accomplishes a definite 

 amount of work. The possibility of this was already given in the tension of the 



