612 CIRCULATION OF BLOOD AND LYMPH. 



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. 



Regulation of the Blood-supply by Changes in the Veins and 

 Capillaries. — The regulation of the circulation by changes in the 

 caliber of the vessels has been treated so far as though such changes 

 occur only in the arteries. The vasomotor system as usually 

 described is, in fact, an arteriomotor system. It affects the arteries 

 and especially the arterioles which stand like flood-gates at the 

 entrance to the capillary bed. As the arterioles are constricted or 

 dilated the flow of blood into the capillary area is diminished or 

 increased. But there is an increasing amount of evidence tending 

 to show that the capillaries and veins are not fixed systems, and 

 that they also are capable of dilatation or constriction under the 

 influence of either local chemical stimuli or of special nerve fibers, 

 which in these cases might be designated as venomotor and capillo- 

 motor fibers. It has long been known that the portal vein has a 

 definite supply of motor nerve fibers and more recently a similar 

 system has been demonstrated for the mesenteric veins. From 

 many sides facts have been discovered which show that the 

 capillaries also are capable of changing their cahber under the 

 direct influence of nerve stimulation and especially as a result of 



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



