THE NERVOUS SYSTEM 7 



nervous organs. These neurones become ac- 

 tive when their sense organs are appropriately 

 stimulated and transmit impulses from the 

 periphery to the central organs. They are, 

 therefore, classed as afferent or sensory neu- 

 rones. Another set of neurones transmit im- 

 pulses from the central organs to the muscles 

 and other mechanisms of response. These are 

 called efferent neurones. It might seem that 

 the afferent neurones leading into the central 

 organs and the efferent neurones leading out 

 from them were all that were necessary for 

 even a complex nervous system, but as a mat- 

 ter of fact there is a host of other neurones 

 which never reach beyond the limits of the 

 central nervous system and which are con- 

 cerned with bringing one part of this system 

 into communication with another. As a rule 

 these neurones cannot be designated appro- 

 priately as either afferent or efferent. They 

 have, therefore, been termed internuncial. In 

 the higher animals they certainly far outnum- 

 ber the afferent and the efferent elements col- 

 lectively and constitute the bulk of the central 

 nervous system in these forms. Thus, not- 

 withstanding the enormous numbers of neu- 

 rones in the higher animals and the intricacy 



