xvi THE NERVOUS SYSTEM 283 



CHAPTER XVI 

 THE NERVOUS SYSTEM 



The frog has the power not only of performing a large 

 number of complicated movements, but of adapting its 

 actions to the various elements of its environment. The 

 initiation and control of these movements are dependent 

 upon the reception of stimuli either from within or without 

 the organism and the transfer of the impulses thus arising 

 to the muscles which by their contraction bring about the 

 required actions. When the frog withdraws its foot when 

 it is irritated, or snaps at a moving insect, it is performing an 

 act of an adaptive nature in response to an external stimu- 

 lus. It is evident that the actions of the frog in relation to 

 external stimuli and the coordination of activities going on 

 in different parts of the organism necessitate some highly 

 specialized means for the transfer and direction of impulses, 

 and it is with these functions that the nervous system is 

 especially and primarily concerned. But the nervous sys- 

 tem has another important function, inasmuch as it affords 

 the means for the accumulation of the effects of experiences 

 whereby the animal is enabled to profit by its former behav- 

 ior and modify its conduct to suit new situations. This 

 latter power forms the basis of intelligence, a faculty rather 

 feebly developed in the frog, it is true, but, as we shall see 

 later, a not unimportant element in the life of the animal. 



The nervous system has often been compared to a system 

 of telegraph wires by means of which any one part of a 

 country may be put into communication with any other 



