THE CIRCULATION. 271 



Ludwig holds that the reduction of blood-pressure in the capillaries 

 of the brain, but particularly those of the medulla, excites the acceler- 

 ators. Oxygen is an accelerator. When the heart beats rapidly 

 from any agreeable cause, or one feels "light at heart," the manifesta- 

 tion is due to the influence of the accelerator fibers on the heart. 



Here a psychical action excites the center of the accelerator. 



Thus, the heart is controlled by two nerves whose functions are 

 diametrically opposite in character. They establish a system of 

 "check" upon one another, each normally preventing extremes in the 

 action of the other. 



Influence of Drugs. Because of the complicated action of various 

 drugs upon the heart, many observers are led to believe that there are 

 various internal mechanisms of the heart upon which these substances 

 act. Besides acting upon the muscular tissue, some are found which 

 exert influences upon the intracardiac nerves. The two drugs that are 

 most familiar to the physiologist and with which he is most engaged 

 in performing his experiments are atropine and muscarine. Their 

 actions are both nervous. Thus, atropine paralyzes the inhibitory post- 

 ganglionic fibers of the vagus, thereby giving the accelerators full sway, 

 the consequence being augmentation of the heart's beats. On the other 

 hand, muscarine stimulates permanently the inhibitory postganglionic 

 fibers so that the heart-beats are slowed, or, if the dose be large enough, 

 complete arrest of heart movement follows. 



If a frog's heart be excised and placed in a suitable vessel and a 

 few drops of a very dilute solution of muscarine be placed upon it, its 

 beats will soon cease and it will continue quiescent as long as the 

 muscarine remains upon it. When the muscarine is removed and 

 atropine applied to the heart, its regular beats manifest themselves 

 within a short time. Pilocarpine also stimulates the cardio-inhibitory 

 postganglionic fibers and slows the heart. Atropine removes this and 

 the heart beats faster. 



Some drugs produce results by their effects upon the heart-muscle 

 alone, either stimulating or depressing the same. Thus, the muscular 

 contractions are rendered more forceful while the rate is uninfluenced 

 by the action of strophanthus, etc. The muscular contractions are 

 depressed by veratrum, aconite, etc. 



In addition to drugs influencing the heart's action by effects upon 

 its muscle and ganglionic nerve terminations, some exert an influence 

 upon the vagus center in the medulla oblongata. Thus, aconite and 

 adrenalin, by stimulating this center, produce a slowing of the heart- 

 beats. 



