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A TEXTBOOK OF PHYSIOLOGY 



than chrono tropic. Its chrono tropic fibres generally do not pass to 

 the siiiu -auricular node; perhaps they may pass to the A.-V. node. 

 The left nerve has been said to act directly on the ventricles; it is 

 probable, however, that the vagus nerves only manifest their action 

 indirectly through action upon the auricle (Figs. 44 and 80). We 

 need not suppose different kinds of nerve endings; it is probable 

 that the different results are obtained through varying intensity 

 of stimulation action upon the same endings. After vagal arrest, the 

 heart beats more forcibly, owing, perhaps, to the greater accumula- 

 tion of contractile material during the period of rest. 



The converse of all these effects occurs on stimulation of. the 

 accelerator nerves (Figs. 78 and 79). During stimulation of these 



-igji 



r //'' ,;' " 



FIG. 82. DISSECTION OF THE VAGUS, THE DEPRESSOR, AND CERVICAL SYMPATHETIC 

 NERVES IN THE RABBIT. (Cyon.) 



nerves the heart beats more quickly and forcibly, excitability and 

 conductivity being also increased. Excitation of these nerves may 

 excite to renewed efforts an excised heart which has just ceased to 

 beat owing to a withdrawal of the supply of nutritive solution ; hence 

 it is thought by some that the accelerator nerves tonically exert a 

 sustaining influence on the heart. The accelerator nerves act directly 

 upon both auricl s and ventricles (Figs. 79 and 80). 



The alkaloid atropine paralyzes the vagal nerve endings in the heart , 

 while nicotine paralyzes the endings of the preganglionic fibres in the 



