158 A MANUAL OF PHYSIOLOGY 



peripheral distribution can, therefore, be answered by observing 

 whether any effect of stimulation is abolished by nicotine. If, for 

 instance, the excitation of a nerve caused constriction of certain 

 bloodvessels before, and has no effect after, the application of 

 nicotine to a ganglion, its vaso-constrictor fibres, or some of them, 

 must be connected with nerve-cells in that ganglion. 



We have thus traced the vaso-motor nerves from the 

 cerebro-spinal axis to the bloodvessels which they control ; 

 it still remains to define the portion of the central nervous 

 system to which these scattered threads are related, which 

 holds them in its hand and acts upon them as the needs of 

 the organism may require. 



Vaso-motor Centres. Now, experiment has shown that there 

 is one very definite region of the spinal bulb which has a most 

 intimate relation to the vaso-motor nerves. If while the blood- 

 pressure in the carotid is being registered, say, in a curarized 

 rabbit, the central end of a peripheral nerve like the sciatic 

 is stimulated, the pressure rises so long as the bulb is intact, 

 this rise being largely due to the reflex constriction of 

 the vessels in the splanchnic area. If a series of trans- 

 verse sections be made through the brain, the rise of 

 pressure caused by stimulation of the sciatic is not affected 

 till the upper limit of the bulb is almost reached. If the 

 slicing is still carried downwards, the blood-pressure sinks, 

 and the rise following stimulation of the sciatic becomes less 

 and less. When the medulla has been cut away to a certain 

 level, only an insignificant rise or none at all can be obtained. 

 The portion of the medulla the removal of which exerts an 

 influence on the blood-pressure, and its increase by reflex 

 stimulation, extends from a point 4 to 5 mm. above the 

 point of the calamus scriptorius to within i to 2 mm. of the 

 corpora quadrigemina (Owsjannikow). Other observers give 

 narrower limits. Stimulation of the medulla causes a rise, 

 destruction of this portion of it a fall, of general blood- 

 pressure. There is evidently in this region a nervous 

 ' centre ' so intimately related, if not to all the vaso-motor 

 nerves, at least to such very important tracts as to deserve 

 the name of a vaso-motor centre. Experiment has shown 

 that this is much the most influential centre, and it is 

 usually called the chief or general vaso-motor centre. But 



