CH. XXI.] CARDIAC SYMPATHETIC NERVES 253 



Influence of Drugs. The question of the action of drugs on the 

 heart forms a large branch of pharmacology. We shall be content 

 here with mentioning two only, as they are largely used for experi- 

 mental purposes by physiologists. Atropine produces consider- 

 able augmentation of the heart-beats by paralysing the inhibitory 

 mechanism. Muscarine (obtained from poisonous fungi) produces 

 marked slowing, and in larger doses temporary stoppage of the 

 heart. Its effect is a prolonged inhibition, and can be removed by 

 the action of atropine. The action of atropine cannot, however, be 

 easily antagonised by muscarine ; a large dose is necessary. That these 

 drugs act on the nerves, and not the muscular substance of the 

 heart, is shown by the fact that in the hearts of early embryos, so 

 early that no nerves have yet grown to the heart, these drugs have 

 little or no effect. (Pickering.) 



Reflex Inhibition. Thus there is no doubt that the vagi nerves 

 are simply the media of an inhibitory or restraining influence over 

 the action of the heart, which is conveyed through them from the 

 centre in the medulla oblongata, which is always in operation. The 

 restraining influence of the centre in the medulla may be reflexly 

 increased by stimulation of many afferent nerves, particularly those 

 from the nasal mucous membrane, the larynx, and the lungs. A 

 blow on the abdomen causes inhibition and fainting ; a blow on the 

 larynx, even a moderate one, will kill. There is no comparison 

 between the ease with which stimulation of the laryngeal or pul- 

 monary fibres produces inhibition, as compared to the difficulty of 

 obtaining inhibition from the alimentary tract. Tobacco smoke in 

 some people and animals, by acting on the terminations of the 

 vagi or their branches in the respiratory system, may also produce 

 reflex inhibition of the heart. Some very remarkable facts concern- 

 ing the readiness by which reflex inhibition of the fish's heart may 

 be produced were made out by M 'William ; any irritation of the tail, 

 gills, mucous membrane of mouth and pharynx, or of the parietal 

 peritoneum, causes the heart to stop beating. 



In connection with the subject of reflex inhibition, it may be 

 mentioned in conclusion that though we have no voluntary control 

 over the heart's movements, yet cerebral excitement will produce an 

 effect on the rate of the heart, as in certain emotional conditions. 



Action of Chloroform on the Cardiac Mechanism. The mammalian 

 heart is more difficult to stop by stimulation of the vagus than the 

 frog's heart ; commonly it is only slowed, and the amplitude of the 

 beat reduced, yet it is most important for the student of medicine 

 to recollect that vagus inhibition may have far-reaching results. 

 One of the most familiar causes of heart stoppage in surgical practice 

 is that produced by chloroform; chloroform acts directly on the 

 cardiac tissue when it is administered incautiously, or in too large 



