THE VASOMOTOR NERVES. 607 



are therefore designated as vasodilator nerve fibers. This discovery 

 was made in connection with the chorda tympani nerve, a branch 

 of the facial, which sends secretory fibers to the submaxillary gland. 

 When this nerve is cut and the peripheral end is stimulated a secre- 

 tion of saliva results and at the same time, as Bernard showed, the 

 blood-vessels of the gland dilate; the flow of blood is greatly in- 

 creased in the efferent vein and may even show a pulse. 



In the nervous regulation of the blood-vessels we have to con- 

 sider, therefore, the existence and physiological activities of two 

 antagonistic sets of nerve fibers: First, the vasoconstrictor fibers, 

 whose action causes a contraction of the muscular coats of the ar- 

 teries and therefore a diminution in the size of the vessels. Second, 

 the vasodilator nerve fibers, whose action causes an increase in size 

 of the blood-vessels, due probably to a relaxation (inhibition) of the 

 muscular coats of the arteries. Before attempting to describe the 

 present state of our knowledge upon these points it will be help- 

 ful to refer to some of the methods by means of which the existence 

 of vasomotor fibers has been demonstrated. 



Methods Used to Determine Vasomotor Action. The 

 simplest and most direct proof is obtained from mere inspection, 

 when this is possible. If stimulation of the nerve to an organ 

 causes it to blanch, the presence of vasoconstrictor fibers is dem- 

 onstrated unless the organ is muscular and the blanching may be 

 regarded as a mechanical result. On the other hand, if stimulation 

 of the nerve to an organ causes it to become congested or flushed 

 with blood the presence of vasodilator fibers may be accepted. It 

 is obvious, however, that this method is applicable in only a few 

 instances and that in no case does it lend itself to quantitative 

 study. 2. Vasomotor effects may be determined by measur- 

 ing the outflow of blood from the veins. If stimulation of the 

 nerve to an organ causes a decrease in the flow of blood from the 

 veins of that organ, this fact implies the existence of vasoconstrictor 

 fibers, while an opposite result indicates vasodilator fibers. 3. 

 By variations in arterial and venous pressures. When vaso- 

 constrictor fibers are stimulated there is a rise of pressure in the 

 artery supplying the organ and a fall of pressure in the veins 

 emerging from the organ. This result is what we should expect if 

 the constriction takes place in the region of the arterioles. The 

 diminution in size of these vessels by increasing peripheral resistance 

 augments the internal pressure on the arterial side of the resistance, 

 and causes a fall of side pressure on the venous side (see p. 515). 

 If the area involved is large enough the increased resistance will 

 make a perceptible difference in pressure, not only in the organ 

 supplied, but also in the aorta; there will be a rise of general (dias- 

 tolic) blood-pressure. On the other hand, a vasodilator action IP 



