CH. XXI.] VASO-MOTOR NERVES 299 



pressure is not affected at all, and the centre can be influenced 

 reflexly by the stimulation of afferent nerves, the pressor and 

 depressor nerves, which we shall be considering immediately. 



After the destruction of the vaso-motor centre in the bulb, there 

 is a fall of pressure. If the animal is kept alive, the vessels after a 

 time recover their tone, and the arterial pressure rises ; it rises still 

 more on stimulating the central end of a sensory nerve ; this is due 

 to the existence of subsidiary vaso-motor centres in the spinal cord ; 

 for on the subsequent destruction of the spinal cord the vessels again 

 lose their tone and the blood-pressure sinks. 



The vaso-motor path is down the lateral column of the spinal 

 cord, and the fibres terminate by arborising around the cells in the 

 grey matter of the subsidiary vaso-motor centres, the anatomical 

 position of which is uncertain, though it is probably in the cells of 

 the intermedio-lateral tract. From these cells fresh axis-cylinder 

 processes originate, which pass out as the small medullated nerve- 

 fibres in the anterior roots of the spinal nerves. 



The vaso-constrictor nerves for the whole body leave the spinal 

 cord by the anterior roots of the spinal nerves from the second 

 thoracic to the second lumbar, both inclusive. They leave the roots 

 by the white rami communicantes, and pass into the ganglia of the 

 sympathetic chain, which lies on each side along the front of the 

 vertebral column. The ganglia on this chain (the lateral ganglia of 

 Gaskell) may also be called the chain of vaso-motor ganglia, because 

 here are situated cell stations on the course of the vaso-constrictor 

 nerves for the head, trunk and limbs. That is to say, the small 

 medullated nerve-fibres terminate by arborising around the cells of 

 these ganglia, and a fresh relay of axis-cylinder processes from these 

 cells carry on the impulses. 



The next figure (fig. 297) represents diagrammatically how this 

 occurs. The sheaths of the fibres are not represented. 



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 corresponding 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 some- 

 where 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 

 annulus of Yieussens to the inferior cervical ganglion, and thence 



