RELATION SUPRARENAL GLANDS TO CIRCULATION 203 



or the sympathetic ganglia. In subsequent experiments it was found pos- 

 sible to bring about dilatation in the limb by the application of epinephrin 

 directly to these ganglia. 



A systematic study was also made of the vasodilator reactions in 

 the various orders of vertebrates. The mechanism was found to be absent 

 in reptiles, birds, and one order of mammals, the rodents (rats and rab- 

 bits). Its presence was demonstrated, however, in one the lower mam- 

 mals, the opossum. It was found present in all of the carnivors examined 

 (cat, dog, ferret). In the cat it appeared, however, the mechanism was 

 not functional until about the eleventh week of life. 



That vasodilatation is a characteristic and, perhaps, the most signifi- 

 cant phase of the vasomotor reaction to epinephrin can no longer be ques- 

 tioned. It seems evident that the injection of any effective quantity 

 stimulates primarily the vasodilator mechanisms. Whether fall of blood- 

 pressure results depends upon the extent to which vasoconstrictor mechan- 

 isms are also brought into play. A considerable amount of direct evidence 

 supports the inference that the dilator mechanisms have a lower threshold 

 for epinephrin than have the constrictor. Even in organs which give well 

 marked vasodilator responses with smaller dosages, constriction appears 

 when the dosage is sufficiently increased. This apparently is always true 

 in case of the isolated organs. When, however, the drug is administered 

 to the intact animal, the vasoconstrictor tendency in organs having the 

 less effective vasomotor mechanisms may be overcome by high systemic 

 blood-pressure and relative vasodilatation produced. 



The term "vasodilator mechanism" as herein employed can as yet 

 be given no very definite meaning. Whether the "mechanism" is one 

 which involves direct final effects or whether it merely inhibits vasocon- 

 strictor mechanisms remains to be determined. In the absence of adequate 

 data on the general physiology of vasodilatation, no convincing inferences 

 can be offered. 



The most that can be definitely said at the present time is that epi- 

 nephrin is neither a vasoconstrictor nor a vasodilator substance, generally 

 speaking, but is either, depending upon the dosage employed, the organ 

 involved and the condition of the organism as a whole. Further data 

 bearing upon the point will be found in succeeding paragraphs. 



Differential Effects of Epinephrin in Various Organs 



In a consideration of the effects of epinephrin upon the circulation, 

 attention is directed primarily to the reactions in the heart and in the 

 peripheral blood-vessels. Of subsidiary but potentially considerable im- 

 portance are the effects upon pulmonary circulation and upon venous 

 blood-pressure. Any material fall in the venous pressure or any consider- 



