NERVE 73 



4. Classification of Neurons by the Direction of Conduction. 



Since a nerve is normally stimulated from either the 

 one or the other end, and hence conducts in one direction, 

 and since the passage of impulses along it is made manifest by 

 changes in the structure to which it goes (p. 59), it is possible 

 to classify nerve fibres according to whether they conduct to or 

 from the central nervous system, and according to the structure 

 upon which they act. 



To find out the direction of conduction and the special 

 mode of action of any nerve, after its anatomical distribution 

 has been determined, two methods of investigation may be 

 employed — 



1st. The nerve may be cut, and the results of section 

 studied. 



2nd. The nerve may be stimulated, and the result of 

 stimulation noted. 



Usually these methods are used in conjunction ; the 

 nerve is cut, and when the changes thus produced have been 

 noted, first the upper or central end and then the lower or 

 peripheral end of the cut nerve are stimulated. 



It is, of course, only if a nerve is constantly transmitting 

 impulses that section reveals any change. If the nerve is 

 not constantly in action, stimulation alone will demonstrate 

 its functions. 



A. Outgoing or Efferent Nerves- — Section of certain nerves 

 produces a change of action in muscles, glands, etc., or, if 

 the nerve is not constantly acting, stimulation of the peri- 

 pheral end of the cut nerve causes some change in the 

 activity of these structures. Stimulation of the central end 

 of such nerves produces no effect. These nerves there- 

 fore conduct impulses outward from the central nervous 

 system. 



The nerves going to the skeletal muscles may cause either 

 an increased activitv or a decreased activitv accordinsf to the 

 way in which they are called into action, so that at one time 

 the nerve may be an excitor of the muscle, at another time 

 an inhibitor. 



The nerves going to the visceral muscles, on the other 



