THE VASOMOTOR NERVES. 621 



this influence, we have the lactic acid produced in muscles during 

 activity and probably also the carbon dioxid produced in this as 

 in other tissues. These substances in physiological concentra- 

 tions cause a relaxation of muscular tonus by virtue of their acidity 

 and, therefore, may act to produce a local dilatation during func- 

 tional activity and thus provide the organ with more blood at the 

 time that it is needed. On the other hand, the internal secretion 

 of the adrenal glands (epinephrin) and possibly also of the infun- 

 dibular portion of the pituitary gland have the reverse effect, 

 causing a vasoconstriction and thus tending to maintain normal 

 vascular tone. Evidence has accumulated in recent years which 

 indicates that the secretion of epinephrin may be increased under 

 various conditions, such as emotional excitement* or sensory 

 stimulation, and it is possible that by this means the arterial 

 pressure may be increased under conditions which in nature 

 might be supposed to call for mental and muscular activity. 

 Therapeutically, various substances may be introduced into the 

 circulation which, by chemical action, cause a constriction or a 

 dilatation of the peripheral arteries and thus raise or lower general 

 blood-pressure. In the former class of vasoconstricting reagents 

 we have such substances as epinephrin, digitalis, etc., while in the 

 latter class the nitrites, especially amyl nitrite (Brunton), have 

 been much used, particularly in such conditions as angina pectoris, 

 in which a quick relief from a state of vascular hypertension is 

 desirable. 



* Cannon, "Bodily Changes in Pain, Hunger, Fear, and Rage," 1915. 



