Chap. XIX] THE NERVOUS SYSTEM 371 



Physiological nerve stimulation. — The best explanation is 

 that the true nature of nerve-impulse is a physical molecular 

 vibration set up either in the nerve-cell or the end organs and 

 transmitted along the nerve-fibre. 



Direction of nerve-impulse. — Within the body nerve-impulses 

 travel in two directions : (1) from the cell-body to the periphery, 

 and (2) from the periphery to the cell-body. 



Afferent and efferent nerve-fibres. — From the previous para- 

 graph it is deduced that the nerve-fibres are divided into two great 

 classes : (1) efferent or centrifugal are those in which the direc- 

 tion for the nerve-impulses to travel is from the cell-body to the 

 periphery; and (2) afferent or centripetal are those in which 

 the impulses travel from the periphery to the cell-body. 



The most striking example of efferent fibres are those which 

 convey impulses that stimulate functional activity, i.e. muscular 

 contraction or glandular secretion ; hence the efferent nerve- 

 fibres are often spoken of as motor, although motion is the mani- 

 festation of but a class of neurones. On the other hand, afferent 

 fibres are often spoken of as sensory, because it is to them that 

 sensation is due. 



Reason for direction of nerve-impulse. — Normally the efferent 

 fibres are stimulated only through the cell-bodies from which 

 they spring, and the afferent fibres are stimulated only at their 

 endings. For this reason a nerve-fibre can carry impulses only 

 in one direction. 



Classification of nerve-fibres. — In addition to classifying nerve- 

 fibres as efferent and afferent, we may subdivide peripheral nerve- 

 fibres into smaller groups depending upon their physiological dif- 

 ferences. Numerous experiments have demonstrated that the 

 effect of an impulse conveyed by nerve-fibres may be either excita- 

 tory or inhibitory; i.e. the tissue or cell may be stimulated to 

 activity, or if already in activity it may be reduced to a condition 

 of rest. On this basis both afferent and efferent fibres may be 

 subdivided into excitatory and inhibitory fibres. Each of these 

 subgroups may be further divided according to the kind of activity 

 it excites or inhibits, and according to the kind of muscle or tissue 

 in which it ends. The following classification, taken from " Text- 

 book of Physiology," by William H. Howell, depends upon three 

 principles : (1) the direction in which the impulse travels normally ; 



