884 PHYSIOLOGY OF DIGESTION AND SECRETION. 



is this action that produces the great rise in blood-pressure, seen 

 especially after the antagonistic influence of the cardio-inhibitory 

 center is thrown out by section of the vagi. Under usual conditions 

 this effect on blood-pressure is short-lasting. The arterial pres- 

 sure after reaching a maximum falls off rapidly to normal, but a 

 new rise can be produced by a new injection. The effect of the 

 epinephrin on blood-pressure is so constant and is produced by 

 such small doses that it may be used quantitatively in estimating 

 the amount of epinephrin in various extracts. Investigation has 

 shown that not all of the arterioles are stimulated to contraction by 

 epinephrin, and the results of many experiments of this kind have 

 led to the generalization (Langley) that the epinephrin acts only 

 upon the plain muscle which receives its innervation from the sym- 

 pathetic autonomic system, and that its effect upon the muscula- 

 ture, whether of stimulation or of inhibition, is the same as that ob- 

 tained by direct stimulation of the sympathetic nerve-supply. This 

 selective action of the epinephrin finds some explanation in the hy- 

 pothesis that it does not affect the muscular substance directly, but 

 acts upon a specialized receptive substance, the myoneural junction 

 (Elliott) at the point where the sympathetic nerve-fiber unites with 

 the muscle-fiber. On the basis of this generalization, perfusion with 

 solutions containing epinephrin has been used as a method for 

 determining whether or not the blood-vessels of any given organ are 

 supplied with vasoconstrictor nerve-fibers. These fibers all belong 

 to the sympathetic autonomic system, and those blood-vessels 

 which exhibit a contraction under the influence of epinephrin prob- 

 ably are supplied with vasoconstrictor nerve-fibers. The use of this 

 method is referred to in connection with the discussion regarding 

 the presence or absence of such nerve-fibers to the arteries of the 

 brain, the lungs, and the heart. On the same generalization it will 

 be noted that plain muscle receiving inhibitory fibers from the 

 sympathetic autonomic system ought to exhibit a relaxation instead 

 of a contraction when acted upon by epinephrin. This, in fact, 

 seems to be the case. The plain muscle in the intestinal walls 

 receives inhibitory fibers from the sympathetic autonomies 

 (splanchnics), and on this musculature epinephrin does cause loss 

 of tone and relaxation instead of contraction, and another beautiful 

 example of the same kind is obtained from the iris. The dilator 

 muscle of the iris is supplied by nerve-fibers from the sympathetic 

 autonomies, and in correspondence with this fact the epinephrin in 

 very minute doses causes a dilatation of the pupil. The blood- 

 vessels of the heart and the skeletal muscles* are said to be dilated 

 by doses of epinephrin which would cause constriction in the arteries 



* Hoskins, Gunning, and Berry, "American Journal of Physiology," 41, 

 513, 1916. 



