250 



PHYSIOLOGY OF THE HEART 



[CH. XX. 



Roots of 

 ^ Vagus 



annulus of Vieussens to the inferior cervical ganglion of the sym- 

 pathetic; fibres from the annulus, or from the inferior cervical 

 ganglion, proceed to the heart (see fig. 258). 



In man, the cardiac branches of the sympathetic travel 

 to the heart from the annulus of Vieussens and cervical 

 sympathetic in superior, middle, and lower bundles of fibres. 

 These pass into the cardiac plexus, and surrounding the coronary 



vessels ultimately reach 

 the heart. They probably 

 contain vaso-motor fibres for 

 these vessels, as well as the 

 more important fibres for the 

 heart itself. 



By stimulating each rootlet in 

 his three groups, Grossmann found 

 the cardio-inhibitory fibres in the 

 lower two or three rootlets of group b 

 and the upper rootlet of group c. 

 There are probably differences in 

 different animals. In the cat and dog 

 Cadman finds that the rootlets in the 

 a group are respiratory and afferent 

 inhibitory, and that all the efferent 

 inhibitory fibres are in group c. 



The inhibitory fibres are medul- 

 lated, and only measure 2/i to 3/u in 

 diameter ; they pass to the heart and 

 have their cell-stations in the ganglia 

 of that organ. The sympathetic 

 fibres, on the other hand, reach the 

 heart as non-medullated fibres ; they 

 have their cell-stations in the sym- 

 pathetic (inferior cervical and first 

 thoracic) ganglia. The augmentor 

 and accelerator centres in the central 

 nervous system have not yet been 

 accurately localised. 



FIG. 257. -Heart nerves of frog. (Diagrammatic.) Influence O/ 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 

 experimental 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 



Ant. foot 



Post, root 



