302 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



higher and higher, and the same result noted, until at last the lower 

 limit of the centre is passed, and the fall of pressure is less and less 

 marked the higher one goes there, until in the animal in which the 

 section is made at the upper boundary of the centre the blood- 

 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 exact position of the vaso-motor centre in the bulb is far from clear ; there 

 is no special group of cells there which an anatomist can point to as exercising 

 this function, in the same way as he can point to the respiratory or the cardio- 

 inhibitory centre. Possibly the cells are scattered over a large area and do not 

 occur in definite groups. 



The fibres that leave these cells to pass down the spinal cord 

 probably travel along the lateral columns; but here again exact 

 information is lacking, and we do not know whether or not they 

 decussate in the bulb or elsewhere. They terminate by arborising 

 around the cells in the grey matter of the subsidiary vaso-motor 

 centres, the anatomical position of which 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 general arrangement of the vaso-motor nerves will have been 

 already gathered from our description of the Autonomic Nervous 

 System (Chapter XVII.); but we may briefly recapitulate the main 

 facts. 



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 of the vertebral column. 

 That is to say, the small medullated or pre-ganglionic 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. 



Those which are destined for the supply of the vessels of the head 

 pass into the ganglion stellatum or first thoracic ganglion, thence 

 through the annulus 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. 



