THE ACTION OF THE HEART 351 



disproves their claim. They put the burden of proof upon the 

 supporters of the "constant stimulus " theory. It must be ad- 

 mitted that at present no conclusive evidence for either view is 

 available, nor are the supporters of either able to picture a satis- 

 factory mechanism of rhythmicity in terms of their particular 

 theory. 



The Extrinsic Nerves of the Heart. The heart, as stated pre- 

 viously, is under the control of the autonomic system. It receives 

 nerve-fibers both from the cranial and thoracico-lumbar systems. 

 The cranial autonomic fibers reach it by way of the tenth cranial 

 nerves, the vagi, the thoracico-lumbar by way of sympathetic 

 ganglia. The vagus nerves give off their cardiac branches in the 

 neck; the cardiac nerves from the thoracico-lumbar system arise 

 from the inferior cervical ganglion, a sympathetic ganglion lying 

 in the lower neck region. Both anatomically and physiologically 

 the two sets of nerve-fibers are distinct. Anatomically the vagus 

 fibers are pre-ganglionic; they arise from cell-bodies in the nucleus 

 of the tenth nerve in the medulla and are myelinated. They 

 terminate about nerve-cells lying on or within the heart itself. 

 The fibers from the sympathetic system are post-ganglionic ; they 

 arise from cell-bodies in sympathetic ganglia, the inferior cervical 

 for the most part, and are non-myelinated. They terminate in 

 the tissues of the heart directly. Since nicotine cuts the connection 

 between pre- and post-ganglionic fibers, application of that drug 

 to the nerve-cells of the heart abolishes the influence of the vagi, 

 but does not affect the thoracico-lumbar control at all. 



Physiologically the vagus fibers are inhibitory; their stimula- 

 tion slows and weakens the heart-beat. When very strongly 

 stimulated they may bring the heart to a complete standstill, 

 although in mammals the standstill is maintained for a few sec- 

 onds only, the heart soon " breaking through" the inhibition. 

 The thoracico-lumbar fibers have precisely the opposite function, 

 being augmentor; their stimulation accelerates and strengthens the 

 beat of the heart. 



In addition to the efferent autonomic innervation just de- 

 scribed the heart is provided with a set of afferent nerve-fibers. 

 These reach the central nervous system either by way of the 

 vagus nerves, or in some species of animals, rabbits for example, 

 as separate nerve-trunks known as the depressor nerves. The 



