THE NERVOUS REGULATION OF THE BLOODVESSELS 183 



tion of this portion of it a severe 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. Some 

 writers prefer to speak of it as the vaso-constrictor centre, since it 

 is undoubtedly connected with most or all of the vaso-constrictor 

 paths, and has not been shown to be similarly connected with the 

 vaso-dilator paths. There is, indeed, not the same solid evidence 

 for the existence of a general vaso-dilator centre in the bulb as for 

 the existence of the general vaso-constrictor centre. Yet there are 

 facts which indicate that the bulbar vaso-motor centre or centres, 

 when reflexly stimulated, can, and often do, respond not merely by 

 an increase or a remission of vaso-constrictor tone, but by a simul- 

 taneous inhibition of vaso-constrictor fibres and excitation of vaso- 

 dilators leading to a fall of pressure, or by a simultaneous inhibition 

 of vaso-dilators and excitation of vaso-constrictors leading to a rise 

 of pressure. 



The spinal cells of origin of the pre-ganglionic segments of the 

 vaso-constrictor paths constitute subordinate centres which either 

 normally support a certain degree of vascular tone, or come to do so 

 after the chief vaso-motor centre has been cut off. 



Thus, in the frog it is possible to go on destroying more and more 

 of the cord from above downwards, and still to obtain reflex vaso- 

 motor effects, as seen in the vessels of the web, by stimulating the 

 central end of the sciatic nerve. Although these effects indeed 

 diminish in amount as the destruction of the cord proceeds, yet a 

 distinct change can be caused when only a small portion of the cord 

 remains intact. 



Similarly, in the mammal evidence has been obtained of the 

 existence of ' centres ' at various levels of the cord, capable of acting 

 eventually, if not at once, as vaso-constrictor centres after the loss 

 of the controlling influence of the bulb. The best example of a 

 vaso-dilator centre is that situated in the lumbar cord, which controls 

 the erection of the penis. After total section of the cord at the upper 

 limit of the lumbar region, erection, which is known to be due to a 

 reflex dilatation of the arteries of the organ through the nervi eri- 

 gentes, can still be caused (in dogs) by mechanical stimulation of 

 the glans penis, so long as the afferent fibres of the reflex arc con- 

 tained in the nervus pudendus are intact. Destruction of the lumbar 

 cord abolishes the effect. It is impossible to avoid the conclusion 

 that a vaso-dilator or erection centre, which is in relation on the 

 one hand with the nervi erigentes, and on the other with the nervus 

 pudendus, exists in the lower portion of the spinal cord. Vaso- 

 motor centres for the hind-limbs have also been located in the 



