300 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



The cell station of any particular fibre is not necessarily 

 situated in the first ganglion to which it passes ; the fibres of the 

 white ramus communicans of the second thoracic do not for 

 instance 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 

 a cell or cells. 



The vaso-constrictor nerves, however, have all cell stations 

 somewhere in the sympathetic system, and the new axis-cylinders 

 that arise from the cells of the ganglia differ from those which 

 terminate there in the circumstance that they do not possess 

 a medullary sheath, but they are pale, grey or non-medullated 

 fibres. Those which are destined for the supply of the vessels 

 of the head and neck pass into the ganglion stellatum or first 

 thoracic ganglion, thence through the annulns of Vieussens to 

 the inferior cervical ganglion, and thence along the sympathetic 

 trunk to their destination. Their cell station is in the superior 

 cervical ganglion. 



Those for the body wall and limbs pass back from the sympathetic 

 ganglia to the spinal nerves by the grey rami communicantes, and 

 are distributed with the other spinal nerve-fibres. The cell stations 

 for the upper limb fibres are in the ganglion stellatum, and for the 

 lower limb fibres in the lower lumbar, and upper sacral ganglia. 



Those for the interior of the body pass into the various 

 plexuses of sympathetic nerves in the thorax and abdomen and 

 are distributed to the vessels of the thoracic and abdominal 

 viscera. This set includes the most important vaso-motor nerves 

 of the body, the splanchnics. Their cell stations are situated in 

 the various ganglia of the abdominal plexuses. 



The vaso-dilatator nerves in part accompany those just 

 described, but they are not limited to the outflow from the second 

 thoracic to the second lumbar. Thus, the nervi erigentes originate 

 as white rami communicantes from the second and third sacral 

 nerves, and the chorda tympani, another good example of a vaso- 

 dilatator nerve, is a branch of the seventh cranial nerve. 



The vaso-dilatator nerves also differ from the vaso-constrictors 

 in not communicating with cell stations in the sympathetic chain : 

 they pass through these ganglia, retaining their medullary sheath, 

 and have their cell stations in the collateral ganglia (such as the 

 semilunar) or in the terminal ganglia on the walls of the blood- 

 vessels themselves. 



All vaso-motor nerves, whether they are constrictor or dila- 

 tator, differ very markedly from the spinal nerve-fibres which are 

 distributed to voluntary muscles in being ganglionated ; that is, 



