212 THE ELEMENTARY NERVOUS SYSTEM 



two classes, primary motor neurones or intermmcial neu- 

 rones. The primary motor neurones, the more primitive 

 of the two, extend from the central organ to a muscle some 

 of whose fibers they control. The internuncial neurones 

 connect one part of the central apparatus with another 

 and their courses, therefore, are entirely within the lim- 

 its of the central organ. The more primitive central 

 nervous organs such as those in the worms consist of lit- 

 tle more than primary sensory and motor neurones. 

 These of themselves constitute a sufficient basis for the 

 simple types of reflex. But besides these there are a few 

 internuncial neurones for internal connections. Above 

 the worms in such forms as the arthropods and mollusks, 

 the internuncial neurones become proportionally more 

 abundant than the other types of neurones and in the ver- 

 tebrates the internuncial type composes the chief mass of 

 the central organs, a feature that reflects the enormous 

 development of associative operations carried out by the 

 vertebrate as compared with the invertebrate. 



But the nervous system of the higher animals is not 

 only a system of tissues derived from a small group of 

 especially sensitive cells associated with a still more 

 primitive element, the musculature. It is a system that in 

 its more differentiated examples has appropriated to itself 

 certain other elements of the body than those that can be 

 looked upon as direct descendants from an original 

 source. In the vertebrates, at least, the primary sensory 

 neurones have in some instances, as in the ear and in the 

 organs of taste, appropriated ectodermic cells from the 

 ordinary integument, and these cells, as taste buds and 

 auditory hair cells, have come to form a constituent part 

 of the sensory mechanism. Without doubt these appro- 

 priated cells add to the effectiveness of the apparatus 



