CENTRAL NERVOUS SYSTEM. 



203 



Figs. 86, 87, the elements which compose it fall into three groups: (1) 

 The afferent neurones; those whose function it is to convey impulses due to 

 external stimuli from the periphery, including the muscles and joints, to the 

 central system. The expression 

 "external stimuli" is in this case 

 intended to include beside those 

 outside of the body, also such 

 stimuli as act within the tissues 

 of the body but outside of the 

 central nervous system ; for ex- 

 ample, those acting on tendons 

 and muscles, and affecting the 

 afferent nerves which terminate 

 in them. The dorsal roots of 

 the spinal cord arise from the 

 cell bodies in the spinal ganglion. 

 Sir Charles Bell (1811) showed 

 that these roots are sensory, since 

 in animals stimulation of the 

 central end of the severed root 

 causes reflex movements and ex- 

 pressions of pain, while in man 

 stimulation of these fibres in the 

 stump of an amputated limb 

 may give rise to all the sensa- 

 tions which would be derived 

 from their stimulation in the 

 normal limb. 



In some vertebrates a few 

 efferent axones leave the spinal 

 cord by the dorsal roots. These 

 fibres can be seen in the chick 

 (Fig. 87). In the frog stimula- 

 tion of the peripheral end of the 

 severed dorsal root may cause 

 contraction of the skeletal mus- 

 cles. 1 There is no good evi- 

 dence, however, that these fibres 

 are present in mammals. 



(2) The ccui ml neurones; 

 those the axones of which never 

 leave the central system, and the function of which is 1<> distribute within 

 this system the impulses which have there been received. 



(3) The efferent neurones; or those the axones of which pass outside of 



1 R. .1. Morton-Smith : Journ. Physiol, vol. xxi. p. L01. 



Fig. s7.— Schema of the distribution of the efferent 

 fibres of the spinal roots: 4, afferent fibres in the dorsal 

 root only ; /•:, /■.', efferent fibres In both dorsal and ventral 

 roots. In the ventral root one group of efferent tiiires 

 goes to m, the striped muscles; another group to ganglion- 

 cells, S, forming a single sympathetic ganglion, or to n\ 

 cells located In more than one sympathetic ganglion, but 

 all connected with one efferent fibre by means of Its col 

 laterals; P, peripheral plexuses into which the axonea 

 hi Bome sympathetic cells run. 



