EFFERENT NERVES 231 



fibers are motor. It will likewise be seen that all sensory fibers 

 are centripetal or afferent, but not all centripetal or afferent 

 fibers are sensory. For impressions made upon the terminations, 

 or upon the trunk, of a centripetal nerve may cause (i) pain, or 

 some other kind of sensation; (2) special sensation, (3) reflex 

 action of any kind; (4) inhibition. Similarly impressions made 

 upon a centrifugal nerve may (i) cause contraction of a muscle 

 (motor nerve) ; (2) influence nutrition (trophic nerve) ; (3) con- 

 trol secretion (secretory nerve); (^inhibit, augment, or stop any 

 other efferent action (Kirkes). 



To these two classes, efferent and afferent, should be added a 

 third, the intercentral fibers which connect different parts of the 

 nervous centers. Most of these even can be called either afferent 

 or efferent. 



Characteristics of Efferent Nerves. In case of these nerves 

 a force is generated in the centers and conveyed by the nerves 

 to the periphery, where it manifests itself in one of the ways 

 mentioned above as characteristic of centrifugal fibers. 

 Division of these fibers, or interference with their conductivity 

 by disease or otherwise, renders impossible the manifestation of 

 nervous force generated in the center, for the simple reason that 

 the organ to which the fibers are distributed cannot receive the 

 message intended for it. For instance, a muscle cannot, by the 

 most persistent effort of the will be made to contract if the motor 

 fibers running to that muscle are divided. In case, however, of 

 division of efferent nerves, if the peripheral end be irritated, thus 

 roughly counterfeiting normal stimulation, the ordinary effects 

 of normal stimulation will be brought about, provided (as is 

 usually the case) that particular nerve can be thus directly stimu- 

 lated. Stimulation, however, of the central end of such a cut 

 nerve produces no effect. No matter whether such efferent 

 nerves receive their stimulus directly from the center or arti- 

 fically, as by mechanical or electrical means, the effect is pro- 

 duced in the end organs, whatever they may be. It is an invari- 



