NERVE-SUPPLY 71 



tions in the heart's activity occur through the intervention 

 of the central nervous system. 



The Efferent Nerves of the Heart 



The heart receives efferent fibres from two sources, the 

 vagus and the sympathetic. 

 The Vagus — 



1. Slows the beat and stops it on strong stimulation; 



2. Diminishes the amphtude of the beat; 



3. Prolongs the auriculo-vcntricular interval, by de- 



pressing the conductivity of the bundle of His. 



The Sym/pathetic, the fibres of which emerge from the 

 upper thoracic segments of the cord, is in every way 

 antagonistic to the vagus. It therefore — 



1. Quickens the beat; 



2. Increases its amplitude ; 



3. Decreases the auriculo-vcntricular interval. 



The centre for the control of the heart resides in the 

 medulla at the nuclei of origin of the vagus. This region 

 probably controls the spinal centres from which the 

 sympathetic fibres issue. 



The efferent nerves may be called into play reflexly by 

 stimulation of sensory nerves, by impulses from the higher 

 centres, and by changes in the blood bathing the centre. 



Cardiac Reflexes 



Stimulation of almost any sensory nerve has the effect 

 of altering the rate of the beat in one direction or the other. 

 The most important reflexes, however, are those arising in 

 the heart itself and in the lungs. 



Reflexes originatmg in the Heart. — The heart is liberally 

 suppUed with afferent fibres, which travel up in the vagus, 

 and probably also in the sympathetic. Arising at the base 

 of the heart, and at the root of the aorta, are the depressor 



