318 MANUAL OF PHYSIOLOGY. 



glion of the sympathetic brought about an opposite result, namely, 

 a loss of temperature and contraction of the vessels on the same 

 side as the stimulus was applied. If the stimulus was much 

 increased, the vessels contracted much more than the normal 

 amount, but on cessation of the stimulus they became greatly 

 dilated above the normal point, and the temperature rose again, 

 but after a time the effect of the stimulus gradually passed off. 

 From this it was concluded that the sympathetic in the neck con- 

 veyed to the muscles in the blood vessels impulses which caused a 

 certain amount of habitual contraction of the vessel wall, which 

 was called tonic contraction, corresponding to what was already 

 recognized as arterial tone. When the nerve was divided this 

 tone disappeared, but when gently stimulated it reappeared, and 

 when more strongly stimulated an exaggerated contraction set in 

 causing complete occlusion of many of the vessels. 



Subsequent experiments have shown that all the vessels of the 

 body are supplied with similar vasomotor 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 me- 

 dulla (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 

 paralysis 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 



