THE SYMPATHETIC SYSTEM 269 



ganglia and in other cases (in the lateral chain only), a 

 single central ganglionic fibre will give birth to successive 

 ganglion-splanchnic fibres which will branch out at various 

 levels in the lateral chain. In this way, both in length 

 and in height, the disposition becomes more complicated. 

 Furthermore, we must know that in height, next to 

 giving off of successive collaterals, which after interruption 

 communicate with ganglion-splanchnic fibres, there exists 

 also a change of plane en block of the central ganglionic 

 fibres. In this way a central ganglionic system gives off 

 sometimes ganglion-splanchnic fibres, sometimes ganglion- 

 somatic fibres. Fibres, which if looked at as a whole, can 

 arise from a lower plane to the original plane of the central 

 ganglionic group. There is, therefore, a change of level 

 from the top and one from the bottom. We shall see that 

 as far as the thoracic and lumbar region is concerned this 

 change of level is not due to chance. 



Finally, and this is an important point, there exists in 

 the lateral chain commissural fibres which unite the 

 various ganglionic centres of the lateral chain. 



This complex structure is found in all superior verte- 

 brated animals. Comparative anatomy reveals that the 

 inferior vertebrates have a much simpler sympathetic 

 system ; the lateral chain only exists as isolated ganglia, 

 separated from each other. Furthermore, let us not forget 

 that this abnormal disposition may also be present in man 

 as shown by Bichat and that we have spaces between the 

 various portions of the lateral column. 



Here then in general is the disposition of the motor arc. 

 Notice that the motor arc has two neurons: one extra 

 axial neuron, the vegetative central ganglionic neuron, and 

 the ganglion-splanchnic or ganglion-somatic neuron. 



