86 THE VITAL FUNCTIONS 



of gravity, and in the burrowing of the roots far into 

 the substance of resistant soil. 



(9) Response to Stimuli. All living protoplasm responds 

 to various stimuli in various ways, or in other words it 

 is sensitive to the forces acting upon it from its sur- 

 roundings. This response depends upon the setting 

 up of processes within the protoplasm which result in 

 some change in the activity of the latter. The most 

 conspicuous expression of response to stimulus is a 

 movement. It is necessary to be clear as to the differ- 

 ence between this kind of movement and the merely 

 passive movement which results from the application 

 of an external force. For instance, if one man pushes 

 another over the edge of a cliff, the falling of the victim 

 is a passive movement, resulting from the force of the 

 push and the force of gravity. But if the intended 

 victim, after receiving the push, saves himself by a 

 sudden leap to one side, that movement is a response 

 to the stimulus of the push. It involves a change in 

 the activity of his nerve and muscle cells. This is a 

 very complicated response in a highly complex organ- 

 ism, though it is not nearly so complicated as the 

 responses we carry out every minute in the course of 

 our daily lives. 



The very simplest organisms, however, exhibit re- 

 sponses to various stimuli, and these are specific, i.e. 

 they vary quite definitely according to the stimulus. 

 It is probable that they can all be explained as con- 

 sisting of a series of purely physical and chemical pro- 

 cesses within the protoplasm, set in motion by the 

 stimulus ; as for instance in the case of the ingestion 

 of food by the protoplasm of the amoeba, which, as we 

 saw, is closely paralleled by the ingestion and ejection 

 of a glass splinter coated with shellac by a drop of 



