684 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



ganglia. Stimulation of the central end of the splanchnic nerves — the 

 spinal cord being destroyed — has no effect upon the hairs, i.e. the solar 

 ganglia and the inferior mesenteric ganglia do not send commissural 

 fibres to the pilo-niotor nerve cells of the vertebral ganglia. 



We find, then, that in a considerable number of cases the nerve cells 

 of one ganglion do not send nerve fibres to the nerve cells of other 

 ganglia. And, in consequence, there cannot be any general system of 

 connection of ganglia with one another by commissural fibres. 



Moreover, whenever an effect is obtained by stimulating the con- 

 necting strand between two ganglia, the effect may always be referred 

 on good grounds to stimulation of fibres other than commissural fibres, 

 namely, either of the peripheral ends of pre-ganglionic fibres, 1 or of the 

 central ends of the pre-ganglionic fibres, 2 or of post-ganglionic fibres. 

 Since, then, commissural fibres are in no case necessary in order to 

 account for phenomena, and in particular cases it can be shown that 

 they do not exist, we may reasonably adopt the view that commissural 

 fibres are absent from the sympathetic system. 



2. It can be shown that the pre-ganglionic fibres, if they excite 

 nerve cells which pass on the stimulus to commissural fibres, can only 

 do so to a very limited degree. From the twelfth thoracic to the 

 lowermost effective spinal nerve (fourth or fifth lumbar), the pre- 

 ganglionic fibres of a given spinal nerve excite the nerve cells of the 

 ganglia below, but do not excite the nerve cells of their own segmental 

 ganglia. 4 Thus, stimulation of the nerve roots of the third lumbar 

 nerve causes erection of hairs in the region of the fourth lumbar 

 ganglion, but none in the region of the third lumbar ganglion, i.e. 

 excitation by pre-ganglionic fibres of the pilo-motor nerve cells of the 

 fourth lumbar ganglion does not set in action commissural fibres 

 passing to any more anterior ganglia above. And similarly with the 

 other ganglia, up to the twelfth thoracic. 



In some other cases the limitation of the stimulus to particular 

 nerve cells is still more striking. In all compound ganglia it is 

 obvious that stimulation of certain of the pre-ganglionic fibres running 

 to the ganglia excites some only of the nerve cells, and no increase in 

 the strength of the stimulus can cause irradiation of nervous impulses 

 to the other cells of the ganglion. And the nerve cells which cannot 

 then be brought into action may be nerve cells of the same class as the 

 cells which are in a state of excitation. Of this we may give an 

 example. In the cat, at times, when the arrangement of nerves is 

 posterior, the fourth lumbar nerve causes erection of hairs on the tip of 

 the tail ; the nervous impulses pass through nerve cells in the coccygeal 

 ganglion ; other nerve cells in the coccygeal ganglia will, on stimulation, 

 cause erection of hairs in the greater part of the rest of the tail ; but 

 no stimulation of the fourth lumbar nerve will affect this region. Hence, 

 pilo-motor nerve cells, set in action by the fourth lumbar nerve, 

 send no commissural fibres to the other pilo-motor nerve cells of the 

 coccygeal ganglion. 



Occasionally facts of the same kind can be shown in simple seg- 

 mental ganglia. Thus in the cat the second and third sacral ganglia 

 contain two sets of vasomotor cells ; one set send their axons to the 

 ano-genital region, and cause contraction of the arteries in this region ; 



1 Cf. p. 634 et seq. " Cf. p. 629. 



a Cf. p. 679 et seq. 4 Cf. p. 634. 



