CHAPTER XIX 

 THE CARDIAC NERVES 



THE vagus nerve, when excited, slows or even arrests the action of 

 the heart (Fig. 44). The cardio-inhibitory nerves, as they are called, 

 have been found in all classes erf vertebrates and in many inverte- 

 brates. The existence of nerve fibres which, when excited, augment 

 and accelerate the beat of the heart has also been demonstrated. 

 These belong to the sympathetic nervous system. In the frog, the 

 two nerves run in one trunk the vago-sympathetic nerve a variable 

 response to stimulation is therefore obtained (Figs. 73, 74). In the 



B 



FIG. 75. CARDIAC NERVES OF FROG. 



rog, the sympathetic fibres come off the ganglion of the third spinal 

 lerve (first post-brachial), and pass along the sympathetic trunk to 

 join the vagus nerve where its ganglion lies at the base of the skull 

 (Fig. 75). In mammals, the accelerator nerves arise from the first 

 to the fifth thoracic anterior spinal nerve roots, the preganglionic 

 ibres having their "cell stations" in the first thoracic and inferior 

 cervical ganglia, whence they pass to the heart partly in company 

 with the cardiac branches of the vagus, and partly as separate 

 twigs (Fig. 76). The vagus cardiac fibres belong to the cerebral 



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