96 NERVOUS SYSTEM OF VERTEBRATES. 



With regard to each of these groups of activities the work of 

 the nervous system is two-fold. On the one hand it receives 

 stimuli from the external world or the internal organs; on the 

 other hand it directs the responses to those stimuli. All actions are 

 in response to stimuli ; directly or indirectly. The stimuli usually 

 affect the organs only through the nervous system. It is noticed 

 that certain characteristic responses habitually follow upon certain 

 stimuli, either as the result of heredity or of individual experience. 

 For example, a frog will jump to catch an object which moves in 

 a manner characteristic of its usual food. So a hellbender will 

 pay no attention to a living earthworm so long as it is quiet, even 

 although the worm be lying on the animal's nose. The moment 

 the worm wriggles vigorously it is snapped up. This is because 

 these animals habitually judge their food by its movements. 

 In other animals the reactions necessary for capturing food are 

 called forth by stimuli received through the organs of smell or 

 taste. In any case the reactions toward the external world are 

 in the nature of reflex acts, simple or complex. As far as the 

 nervous processes are concerned the internal reactions are of the 

 same character, although their reflex nature is not so readily seen 

 as in the case of external activities. 



With regard to the stimuli, then, those which affect the bodily 

 welfare of the animal in its surroundings must be distinguished 

 from those which affect the internal activities. A morphological 

 distinction has long been made between the soma and the viscera. 

 The soma comprises those organs (skin, muscle, skeleton, etc.) 

 by which the animal deals with its environment. The viscera 

 comprise those organs (alimentary canal and appendages, cir- 

 culatory, excretory and reproductive apparatus) concerned with 

 the processes of metabolism by which the organism is built up 

 and the reproductive elements are formed. A part of the soma, 

 the skin, comes into contact with the external world and offers 

 the medium by which stimuli reach the nervous system. The 

 remainder of the soma (muscles and skeleton) responds to these 

 stimuli by appropriate movements under the direction of the 

 nervous system. The peripheral nerve fibers and the central 

 mechanisms which have to do with stimuli affecting the welfare 



