Till; \KKVOUS SYSTEM AND ITS ( '< ).\SKK\ "AT ION 



dilator art i<ni on the part of particular nerves. There 

 i- -ufHcient reason for the belief that in many mixed nerves 

 vasodilator fiber- really arrimipany the va-oron-t rictor-. 

 but that the working of the first class is ma-ked in our 

 ordinary laboratory trials by the greater effect of their 

 antagonists. When we stimulate a nerve \ve are obliged 

 to impress the stimuli upon all the fibers in it ; in the living 

 body there is no such necessity. 1'nder conditions spe- 

 cially devised it is possible to obtain results \vhich lead 

 to the view that vasodilators are widely distributed and 

 bear a considerable part in the regulation of the circulation. 

 Their existence in connection with skeletal muscles, "'lands, 

 the skin of the face, and the tissues of the genitals is well 



recognized. 



We are. therefore, to think of the nervous system as 

 having two routes by which it can influence the beat of the 

 heart and two by which it can modify the decree of con- 

 traction of the blood-vessel.-. There is the possibility of 

 inhibition of the heart through the vagi and the ]x>ssil>il- 

 ity of inhibiting the tone of blood-vessels through the 

 dilators. The heart can be driven to greater contractile 

 activity by means of its accelerators and the blood-vessels 

 can be contracted in a greater degree by the vasocon- 

 strictors. The correspondence is helpful, even though we 

 have to do in the one case with rhythmic, and in the other 

 with sustained, action. But it is only fair to add that in 

 the case of the heart the inhibitory mechanism appears 

 to be more important than its antagonist, while the vaso- 

 constrictors seem to have a more conspicuous influence 1 

 than do the vasodilators in securing the proper distribu- 

 tion of the blood. In short, the heart is most of the time 

 inhibited to some extent, while the blood-vessels are most 

 of the time definitely contracted. Their condition is such 

 that they may be contracted more, or more relaxed, as a 

 re-nit of variation- in the activity of the vasoconstrictor 

 center. 



The chief value of the nervoii- equipment of the heart 

 must lie in making it possible to -peed up the circulation 



