THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 735 



the excitatory action first. Immediately after the intravenous injection 

 of as small an amount as '00008 milligrams per kilogram of body weight, 

 a distinct rise in arterial blood pressure may be observed. It is quite def- 

 inite with '00008 milligrams per kilogram, and is accompanied by a 

 slowing of the pulse. This slowing is caused by stimulation of the .vagus 

 center, as is evidenced by the fact that if the vagus nerves are cut, or 

 sufficient atropine administered to paralyze them, the same dose of 

 epinephrine produces not a slowing but a quickening of the pulse, and 

 consequently a much greater rise in blood pressure. The vagus action 

 is developed not because of an effect of epinephrine on the vagus center, 

 but secondarily because of the rise in blood pressure. 



These preliminary experiments indicate that the locus of action of 

 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 be 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 plethysmo graph 

 during the injection of epinephrine, when a great diminution in volume, 

 accompanying the rise of arterial blood pressure, will be observed. The 

 vasoconstricting effect of epinephrine does not become developed on the 

 large blood vessels near the heart on account of the deficiency in muscu- 

 lar tissue in their walls. Indeed, these vessels may become passively 

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

 ferent parts of the circulation are not equally sensitive to epinephrine; 

 those of the splanchnic area are most sensitive, whereas those of the 

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

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

 to epinephrine. 



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

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

 nephrine still acts, 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 Avails of the blood vessels, 

 but this does not appear to be the case, for it has been found that epi- 

 nephrine is incapable of acting on tissues which are devoid of sympathetic 

 nerve fibers, and is also inactive on those tissues in the embryo which have 

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

 acts only on blood vessels that are supplied by the sympathetic nervous 

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

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



