876 PHYSIOLOGY OF DIGESTION AND SECRETION. 



its functions may be supplied more or less perfectly by the outly- 

 ing cells belonging to the same tissue. 



Physiological Action of the Epinephrin. As stated above, 

 solutions of epinephrin give a marked effect upon the heart-rate 

 and blood-pressure. The great slowing of the heart-rate is due to 

 an action of the epinephrin on the cardio-inhibitory center, since 

 it disappears when the vagi are cut or after the administration of 

 atropin. When this influence of the cardio-inhibitory center is 

 removed it can be shown that the epinephrin causes a stronger and 

 more rapid contraction of the heart, and a similar effect can be ob- 

 tained upon the isolated heart maintained by an artificial circula- 

 tion. It seems probable that this effect of the epinephrin is due to 

 a stimulating action upon the endings of the accelerator nerve in 

 the heart. In addition to the heart effect the epinephrin causes a 

 strong contraction of the arterioles in certain areas, and experiments 

 demonstrate that this effect is due mainly to a peripheral action. It 

 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 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 probably 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 



