THE CARDIAC NERVES 



173 



excited reflexly by irritation of the abdominal nerves e.g., a blow 

 in the abdomen and by increased pressure in the cerebral vessels. 

 The accelerator and augmenting fibres likewise have their centre in 

 the spinal bulb, and this is in tonic action, antagonizing more or less 

 the action of the vagal centre. The vagus nerve, by its action, pro- 

 duces changes which result in a depression of the excitability, the 

 conductivity, the force, and the frequency of the heart. By some 

 authorities these are believed to be separate influences, so that the 

 vagus nerves are said to contain bathmotropic, dromotropic, inotropic, 



Fia. 77. THE EFFECT OF EXCITATION OF THE PERIPHERAL END OF THE VAGUS 

 NERVE UPON THE BLOOD-PRESSURE IN THE AORTA (Top CURVE) AND THE VENA 

 CAVA (SECOND CURVE) IN MM. Ho OF A CURARIZED ANIMAL WITH ARTIFICIAL 

 RESPIRATION. (L. H.) 



Note the inhibition of the heart ; the great fall of aortic and the insignificant rise of 

 vena cava pressure; the escape of the heart from the vagus action and the after- 

 effect on the aortic pressure. The time is marked in seconds, and the signal line 

 shows the duration of vagus stimulation. 



and chronotropic fibres, influencing through their nerve endings the 

 above-mentioned properties in the order named. Possibly, also, the 

 tonicity of the heart muscle is affected. The chronotropic action, 

 slowing the frequency of the beat, is the most characteristic action. 

 The heart becomes dilated and engorged with blood, stopping in 

 diastole. The right vagus, in which chronotropic fibres chiefly run, 

 probably manifests its action upon the auricles through the sinu- 

 auricular node. The left vagus is held by those authorities who 

 believe in the different kinds of fibre to contain chiefly fibres other 



