198 THE AUTONOMIC NERVOUS SYSTEM [CH. XYII. 



involuntary muscles. There is thus an extra cell-station or synaptic 

 junction outside the central nervous system altogether. The 

 autonomic path, in other words, consists of two neurons ; one from 

 the central nervous system to the ganglion, and a second from the 

 ganglion to the peripheral tissue. The first axon is termed the pre- 

 ganglionic fibre; the second, the post-ganglionic fibre. The pre- 

 ganglionic fibres are fine medullated ones, and the post-ganglionic 

 fibres are usually non-medullated, but there are exceptions to this 

 rule. 



The small medullated or pre-ganglionic fibres arise from the 

 following four regions of the central nervous system. 



1. From the mid-brain, issuing therefrom by the third cranial 

 nerve (motor oculi). 



2. From the medulla oblongata, issuing therefrom in the seventh 

 (facial), ninth (glosso-pharyngeal), tenth (vagus), and eleventh (spinal 

 accessory) nerves. 



3. From the thoracic region of the spinal cord, issuing therefrom 

 in the anterior roots of the spinal nerves and passing from these by 

 the white rami communicantes to the sympathetic ganglia. These 

 occur in all the nerves from the first or second thoracic, as far 

 down as the second, third, or fourth lumbar nerves. 



4. From the sacral region of the spinal cord, issuing therefrom by 

 the anterior roots of the second, third, and fourth sacral nerves, and 

 thence passing by white rami communicantes to sympathetic 

 ganglia ; these fibres supply the descending colon, rectum, anus, and 

 urino-genital organs in the pelvis, and they constitute the nervi 

 erigentes. 



It will be noticed that in the spinal district white rami com- 

 municantes only occur in certain regions : but all the spinal nerves 

 have grey rami which consist of post-ganglionic fibres returning to 

 the spinal nerves for distribution to the blood-vessels of the body 

 wall, to the muscles which erect the hairs (pilo-motor nerves), and to 

 the sweat glands of the skin. 



The general arrangement of such nerves is represented in fig. 188. 



The cell-station of any particular pre-ganglionic fibre is not 

 necessarily situated in the first ganglion to which it passes; the 

 fibres of the white ramus communicantes of the second thoracic nerve, 

 for instance, do not all have their cell-stations in the second thoracic 

 ganglion, but may pass upwards or downwards in the chain to a 

 more or less distant ganglion before they terminate by arborising 

 around its cells. It therefore follows that fibres that leave any 

 given spinal nerve by its white ramus, do not necessarily return as 

 post-ganglionic fibres by the grey ramus to the same spinal nerve, 

 although, for the sake of simplifying the diagram, they are repre- 

 sented as doing so in fig. 188. 



