

THE CIRCULATION. 327 



vasodilation, while to a nerve in the fresh state the same irritant 

 produces a primary vasoconstriction. 



By variation in the frequency and strength of the irritation 

 there is afforded a means to differentiate the two kinds of nerves 

 which may traverse the same nerve trunk. The vasodilators are ex- 

 cited by weak currents and slow rhythm. The vasoconstrictors are 

 irritated by stronger currents and greater frequency of irritation. 



Theory of Vasodilator Action. The vasodilators act upon the 

 circular arterial muscle directly. How they act is still hypothetical. 

 Since physiologists know of no muscle through whose contraction 

 the blood-vessels become more dilated, it is assumed that vasodilation 

 is due to a paralysis of the circular fibers of the vessels. That is, the 

 dilators must be inhibitory or vaso-inhibitory nerves. 



The tonus of a blood-vessel depends partly upon impulses from 

 the central nervous system via the vasoconstrictors. It is upon the 

 circular muscles that the dilators are supposed to exert an inhibitory 

 action. 



Frequent allusions, during the discussion of the vasomotor sys- 

 tem, have been made to the effects of experiments upon various vaso- 

 motor nerves. They have been nearly all performed upon animals, 

 and consist, in the main, of section and excitation of various kinds: 

 electrical, thermal, etc. By these means much has been learned con- 

 cerning this very important system important to the physician as a 

 means of explaining many pathological conditions. 



Vasomotor Centers. The main vasomotor center lies in the floor 

 of the fourth ventricle in its gray matter. It is located on each side 

 of the median raphe, and extends three millimeters from a little 

 above the nib of the calamus scriptorius to near the corpora quad- 

 rigemina. Its position was determined by noting that when it was 

 destroyed there was a lack of tonicity displayed by all of the arteri- 

 oles, with a consequent fall in blood-pressure. When this same area 

 was stimulated all of the arterioles were constricted, giving a rise 

 in blood-pressure as a sequel. Section of the cervical spinal cord 

 permits all the arterioles to dilate as the main vasomotor center has 

 been cut off and the blood-pressure falls to 10 millimeters. 



SPINAL VASOMOTOR CENTERS. Experiments demonstrate that 

 with the destruction or paralysis of the main center there results a 

 drop in blood-pressure ; if, however, the animal be kept alive by arti- 

 ficial respiration, after a variable length of time the arterioles regain 

 their tonicity and there is a corresponding rise in pressure. This 

 phenomenon is accounted for by the presence of minor or subsidiary 



