82 VETERINAKY PHYSIOLOGY 



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 according 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- OP 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 peripheral 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 

 activity. The former class may be called augmentop nerves, 

 the latter inhibitory nerves. 



The augmentor nerves may further be divided into groups 

 according to the structures upon which they act. Those act- 

 ing on muscle may be called motor nerves; those acting to 

 cause secretion from a gland, secretory nerves; those acting 

 to constrict blood vessels, vaso-constrictor nerves. 



The inhibitory nerves may be similarly subdivided into mus- 

 culo-inhibitory, secreto-inhibitory, and vaso-inhibitory nerves. 



Ingoing OP Afferent Nerves. Section of another set of 

 nerves may produce loss of sensation in some part of the 

 body. When the peripheral end of the cut nerve is stimu- 

 lated no result is obtained. When the central end is stimu- 



