252 PHYSIOLOGY OF THE HEART. [CH. xx. 



In man, the cardiac branches of the sympathetic probably 

 originate in the same way; they 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. 



The course of the inhibitory fibres in mammals has been recently 

 investigated by Grossmann. He divides the rootlets that leave the medulla 

 to form the ninth, tenth and eleventh cranial nerves into three groups, 

 a, b and c ; a corresponds fairly well with the fibres of the glossopharyngeal, 

 b with those of the vagus, and c with those of the spinal accessory. By 

 stimulating each rootlet he 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 medullated. and only measure 2/j. to 3/x in dia- 

 meter ; they pass to the heart and lose their medulla in the ganglia of that 

 organ. The sympathetic fibres on the other hand reach the heart as non- 

 medullated fibres ; having lost their medulla in the sympathetic (inferior 

 cervical and first thoracic) ganglia. The augmentor centre in the central 

 nervous system has not yet been accurately localised. 



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 experimental purposes by physiologists. Atropine produces 

 considerable augmentation of the heart-beats by paralysing the 

 inhibitory mechanism. Muscarine (obtained from poisonous fungi) 

 produces marked slowing, and in larger doses stoppage of the 

 heart. It produces a similar effect to that of prolonged vagus 

 stimulation, and, as in that case, the effect can be removed by 

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

 be antagonised by muscarine. 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 inhibitoiy 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 almost any 

 afferent nerve, particularly of the abdominal sympathetic, so as 

 to produce slowing or stoppage of the heart, through impulses 



