222 SPECIAL PHYSIOLOGY. 



hence it is called a centripetal action, and the fibre is spoken of as an 

 afferent nerve-fibre; on the other hand, when the central extremity of 

 another kind of nerve-fibre receives the stimulus, the effect of this is 

 propagated outwards towards a muscle; that is to say, a centrifugal 

 action takes place, and the fibre is called an efferent fibre. The effer- 

 ent fibres terminate in muscles, and convey the effects of motorial 

 stimuli ; hence they are called motor or motory fibres. The afferent 

 fibres have received two different names, according to the different 

 offices which they serve. First, some afferent fibres convey the effect 

 of impressions to certain parts of the gray matter of the nervous 

 centres, and then, by a reflected action, which always takes place 

 through gray matter, stimulate certain efferent or motor fibres, which, 

 in turn, excite definite muscles to contract; such a mode of action of 

 the nervous system, is called a reflex action, and the afferent fibres 

 concerned in it, may be called reflex afferent fibres, and the efferent 

 fibres concerned, reflex efferent or reflex motor fibres. The entire 

 nervous apparatus employed in these reflex actions, viz., the afferent 

 fibres, the gray nervous centre, and the efferent fibres, is also spoken 

 of as an excito-motor nervous apparatus; and the phenomena resulting 

 from its action, are named excito-motor phenomena, or reflex acts. 

 Secondly, other afferent fibres convey the effects of impressions or 

 stimuli upon them, to the common sensorium, and there produce sen- 

 sations proper ; these are called sensory afferent fibres, or simply, 

 sensory fibres. 



There is no anatomical difference discernible between the sensory, 

 reflex, and motor fibres. Even between the nerves of common sensa- 

 tion, and those of the different special senses, there is no recognizable 

 distinction, excepting as regards the comparative fineness of the fibres 

 of the latter; but they are connected with different portions or masses 

 of the gray matter of the sensorium. It is presumable that the differ- 

 ence in the functions of afferent and efferent fibres, depends on the 

 direction in which, during life, the effects of stimuli are practically 

 made to operate, and on the difference between the parts to which 

 those effects are ultimately conveyed. In a motor fibre, the stimulus, 

 or rather some state of the nerve-fibre produced by it, travels outwards 

 to a contracting muscle; in an afferent reflex fibre, inwards to a reflex 

 nervous centre; and in a sensory fibre, inwards to a sensitive nervous 

 centre. Sometimes in the living body, a nerve is composed entirely 

 of efferent or motor fibres; for example, the sixth cranial nerve. At 

 other times, a nerve is composed entirely of afferent fibres ; and of 

 these, either the greater part may be purely sensory, as in the case of 

 the nerves of the special senses of sight and hearing, viz., the optic 

 and auditory nerves; or there may be, with the sensory fibres, many 

 afferent reflex fibres, as in the case of the sensory branches of the 

 first and second divisions of the fifth cranial nerve. More commonly, 

 both efferent and afferent fibres, that is motor, sensory, and reflex, are 

 combined together in the trunk of a nerve, as in the case of the third 

 division of the fifth cranial nerve, and of all the spinal nerves. As 

 we shall hereafter see, the afferent and efferent fibres, in this last case, 



