116 MEDULLA 



application of cocaine. This action is due to the paralysis of 

 the antagonistic fibers from the superior cervical ganglion by 

 the cocaine, for the effect is not seen after extirpation of the 

 ganglion. Caffeine is also an antagonist to the stimulating 

 effects of epinephrine on the blood vessels, heart, and uterus. 



EFFECTS OF EPINEPHRINE ON THE CIRCULATION 



The stimulating effects of epinephrine on the mammalian 

 heart may be demonstrated on the isolated organ or on strips 

 of cardiac muscle. Addition of epinephrine to the perfusing 

 fluid results in a marked acceleration of the pulse rate, an in- 

 crease in the strength of beat, and an increased rate of conduc- 

 tion when electrically stimulated. These stimulating effects 

 are more pronounced in a heart poisoned by potassium chloride, 

 chloroform, atropine, cyanide, etc. After chloral poisoning, on 

 the other hand, epinephrine is ineffective in stimulating the 

 cardiac contractions. 



When administered intravenously to mammals, the prelimi- 

 nary acceleration of the pulse is followed by a marked slowing 

 and often by an irregularity in rate. This slowing is brought 

 about reflexly by impulses arising from the elevated blood pres- 

 sure in the vagus center, for it can be abolished by previous ad- 

 ministration of atropine. In chloroform poisoning, epineph- 

 rine still reduces the pulse rate even after vagotomy. This 

 is probably due to stimulation of the vagal endings. 292 This 

 reflex slowing of the heart may even result in complete cessa- 

 tion of the auricular beats due to inhibition by the cardio-in- 

 hibitory center. If the vagi are cut or paralyzed by atropine 

 the inhibitory effects are prevented, and the heart after epineph- 

 rine injections maintains its more forceful and rapid rate of 

 contraction. This cardiac activity still further increases the 

 blood pressure. With large doses of epinephrine the heart 

 becomes dilated to such an extent as to be fatal unless the 

 pericardium be opened. 650 ■ 639 



Epinephrine increases the irritability of the heart. This in- 



