COMMUNICA TION OF NEB VE- CENTRES. 199 



but stimulation 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 alLthe 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 either purely afferent or efferent, and the mixed char- 

 acter which their trunks exhibit is due to cross-branches 

 with neighboring nerves, in which afferent and efferent 

 fibres are interchanged. The olfactory, optic, and audi- 

 tory nerves remain, 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 3STerve-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 physiological character of a nerve-fibre is its con- 

 ductivity its power of propagating a disturbance when 

 once its molecular equilibrium has been upset at any one 

 point it is obvious that through the nervous system any 

 one part of the Body, supplied with nerves, may react on all 

 other parts (with the exception, of such as hairs and nails 

 and cartilages, which are not known to possess nerves) and 

 excite changes in them. Pre-eminently the nervous system 



