THE ENDOCRINE ORGANS, OB DUCTLESS GLANDS 



the excitatory action first, [mmediately after the intravenous injection 



of as small an ;i mount as noons milligrams per kilogram of body weight, 



a distincl rise in arterial hi I pressure may I"- observed. When the 



pise is distinct, it is accompanied by a Blowing of the pulse. This Blow- 

 ing is caused l>.\ stimulation of the vagus center, as is evidenced by the 

 Fad thai it* the vagus nerves are eut, or sufficienl atropine administered 

 to paralyze them, the Bame dose of epinephrine produces not a Blowing 

 hut a quickening of the pulse, and consequently a much greater rise in 

 blood pressure. The vagus action is developed not because of an effecl 

 of epinephrine <>n the \a'_ r us center, bul secondarily because of the rise 

 in blood pressure. 



These preliminary experiments indicate that the locus of action 

 epinephrine, so far as the circulatory system is concerned, is mainly on 

 the small blood vessels, constricting them and thus raising the peripheral 

 resistance. This conclusion can readily ho confirmed by applying the 

 epinephrine directly to the blood vessels of the exposed mesentery, or 

 by enclosing a vascular organ such as the kidney in a plethysmograph 

 during the injection of epinephrine, when a greal diminution in volume. 



accompanying the rise of arterial hi 1 pressure, will be obs< i. Tim 



vasoconstricting effect of epinephrine dees not become developed on the 

 Large blood vessels near the hear! on account of t ho deficiency in muscu- 

 lar tissue in their vails. Indeed, these vessels may hecome passively 

 dilated because of the increased hlood pressure. The arterioles of dif- 

 ferent parts <>f the circulation are not equally sensitive to epinephrine; 

 those of the splanchnic area are mosl sensitive, ,wh< those of the 



heart the coronary vessels do not respond at all in most animals 



page 257). The pulmonary and cerebral \essels have a variable reactivity 

 to epinephrine. 



The effect on the vessels persists after complete destruction, not only 

 of the central nervous system, hut also of the vasomotor nerves; epi- 

 nephrine still ads, for example, on vessels the nerve fibers of which 

 have been allowed to degenerate by cutting them several days before the 

 epinephrine is applied. This would seem to indicate that the epinephrine 



acts directly on the muscular tissue in the walls of the hlood \ 



hut this does not appear to he the for it has been found that epi- 



nephrine is incapable of acting on t a bich are devoid of sympathi 



nerve fibers, and is also inactive on those • u the embryo which 1. 



not yet received any nerve supply. In brief, then, although epinephrine 

 acts only on blood vessels that an- supplied by the Bympatheti( 

 system, it is not on the nerve fibers that the epinephrine unfolds its 

 action. We shall see immediately that this conclusion is in conformity 



