VASO-MOTOR NERVES. 313 



Subsequent experiments have shown that all the vessels of the 

 body are supplied with similar vaso-motor nerves, section of 

 which destroys their tone, while their stimulation causes contrac- 

 tion of all the vessels in the territory presided over by the stimu- 

 lated nerve. 



Experiment has also shown that these nerves come from the 

 cerebro-spinal axis, passing out from the spinal cord as " commu- 

 nicating nerves," commonly becoming associated with the sym- 

 pathetic chain, and are distributed to the vessels either as special 

 nerves, branches of the sympathetic (as the splanchnics), or with 

 the general peripheral nerve trunks. 



The nerve centre, which governs the vast majority of the vaso- 

 motor channels, lies in the upper part of the medulla oblongata 

 in the floor of the fourth ventricle. This is proved by two facts: 

 1st, most of the brain may be removed without diminishing the 

 arterial tone ; and 2d, if the spinal cord be cut below the medulla 

 (artificial respiration of course being kept up) the mean blood 

 pressure is found to fall immediately almost to zero, which is due 

 to the relaxation of the smaller arteries consequent on the paral- 

 ysis of their muscular coat. 



The same can be seen in the web of a frog, in which the me- 

 dulla has been destroyed (pithed) while the circulation is being 

 studied. The small arteries dilate and the pulse becomes apparent 

 in the capillaries, and even in the veins. From these facts it 

 seems highly probable that in the medulla oblongata a vaso-motor 

 centre exists, which regulates the contraction of all the vessels, 

 and keeps them constantly more or less contracted ; the centre 

 receiving some continuous stimulation, which results in a slight 

 general vascular constriction or arterial tone. The existence of 

 such a centre in the medulla, and of nerve channels in the cord 

 leading from such a centre, is made certain by the fact, that if a 

 gentle stimulus be applied to a certain part of the medulla, or 

 just below it, simultaneous general vascular constriction sets in, 

 and is indicated by a great and sudden rise in the blood pressure. 



The action of the vaso-motor centre can be increased, and 

 thereby the tone of the vessels elevated, and the pressure raised, 

 either by (1) direct or (2) reflex excitation. If the blood flowing 



