84 HUMAN PHYSIOLOGY 



in the middle, the change travels in both directions, 



although its result is made manifest by the action of the 

 structure on which it normally acts. This two-way con- 

 duction may also be demonstrated by the experiment of 

 paradoxical contraction, in which by stimulating the branch 

 of the sciatic nerve of the frog going to the muscles of 

 the thigh, the nerve fibres to the gastrocnemius lying 

 alongside of them are also stimulated, and cause that 

 muscle to contract. (Practical Physiology, Chap. VIII.) 



5. Classification of Neurons. Since a nerve is normally 

 stimulated from one or other end, and hence conducts in one 

 direction, and since the passage of impulses along it are made 

 manifest by changes in the structure to which it goes, it is 

 possible to classify nerve fibres according to whether they 

 conduct to or from the central nervous system, and accord- 

 ing to the structure upon which they act. 



To find out the direction of conduction and the special 

 mode of action of any nerve, two methods of investigation 

 are 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 ; first, the 

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

 noted, the upper end and the lower 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 active, stimulation alone will teach anything 

 of its functions. 



Outgoing or Efferent Nerves. Section of certain nerves 

 produce a change of action in muscles, glands, &c., 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 therefore 

 conduct impulses from the central nervous system outward. 



Many of these nerves produce an increase on the activity 

 of the parts to which they go, but others diminish or inhibit 



