GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 207 



scope fails to distinguish an afferent from an efferent fibre, 

 but is readily proved by a simple experiment. If an anterior 

 root be cut and its outer end stimulated, the muscles of the 

 parts to which the trunk which it helps to form is distributed 

 will be made to contract, and the skin will be made to sweat 

 also if the root happen to be one that contains secretory 

 fibres for the sweat-glands. On the other hand, if the cen- 

 tral end of the root (that part of it attached to the cord) be 

 stimulated no result will follow, showing that the root con- 

 tains no sensory, reflex, or excito-motor fibres. With the 

 posterior roots the reverse is the case: if one of them be 

 divided and its outer end stimulated, no observed result fol- 

 lows, showing the absence of all efferent fibres; but stimula- 

 tion of its central end will cause either signs of feeling, or 

 reflex actions, or both. We might compare a spinal nerve- 

 trunk to a rope made up of green and red threads with at 

 one end all the green threads collected into one skein and 

 the red into another, which would represent the roots. At 

 its/farthest end we may suppose the rope divided into finer 

 cords, each of these containing both red and green threads, 

 down to the very finest branches consisting of only a few 

 threads, and those all of one kind, either red or green, one 

 representing efferent, the other afferent, fibres. 



The Cranial Nerves. Most of these are mixed also, but 

 with one exception (the fifth pair, the small root of which is 

 efferent and the large gangliated one afferent) they do not 

 present distinct motor and sensory roots, like those of the 

 spinal nerves. At their origin from the brain most of them 

 are purely afferent or purely efferent, and the mixed character 

 which their trunks exhibit is due to cross-branches with 

 neighboring nerves, in which afferent and efferent fibres are 

 interchanged. The olfactory, optic, and auditory nerves re- 

 main, however, purely afferent in all their course, and others, 

 though not quite pure, contain mainly efferent fibres (as the 

 facial) or mainly afferent (as the glosso-pharyngeal). 



The Intercommunication of Nerve-Centres. From the 

 anatomical arrangement of the nervous system it is clear that 

 it forms one continuous whole. No subdivision of it is 

 isolated from the rest, but nerve-trunks proceeding from the 

 centres in one direction bind them to various tissues and, 

 proceeding in another, to other nerve-centres, which in turn 

 are united with other tissues and other centres. Since the 



