CHAPTER VI. 



THE INHIBITORY NERVES TO THE VASCULAR SYSTEM. 



I TURN now to the question of inhibitory nerves to the vascular 

 system, and I will consider in the first place the innervation of 

 the heart. It is universally recognized that the cardiac fibres 

 of the vagus nerve are inhibitory in action, that they enter the 

 heart as medullated fibres and connect with nerve cells in the 

 heart itself. After nicotine has been given these fibres no longer 

 produce any effect, though at the same time the non-medullated 

 augmentor nerves, which reach the heart from sympathetic 

 ganglia, are still able to cause the same action as before the nico- 

 tine was given ; from this fact the conclusion may be definitely 

 drawn, that all the nerve cells in the heart belong to the vagus 

 group and none to the sympathetic. The action of nicotine thus 

 enables us to draw a conclusion similar to that in the case of 

 Auerbach's plexus in the small intestine ; all the cells belong to 

 the vagus nerve and none to the sympathetic. 



Further the investigations of Dogiel, by means of methylene 

 blue staining, on the nerve structures in the heart show that these 

 medullated fibres connect up with various groups of ganglion 

 cells in the heart by means of collaterals, so that the cardiac 

 vagus fibres fall into line with the rest of the connector nerve 

 fibres belonging to the involuntary system, and are in reality con- 

 nector fibres to the cardiac nerve cells. 



Finally His junior has shown that in an early condition of 

 the embryo the cardiac nerve cells have not yet reached the heart 

 but are found outside it ; with the growth of the heart they be- 

 come included in it. He concludes that they have passed out 

 from the central nervous system to reach the heart, and are not 

 found in the heart itself from the beginning. These cardiac nerve 

 cells behave then in all respects like the sympathetic nerve cells, 

 and demonstrate the uniformity of origin of all the peripheral 

 nerve cells belonging to the involuntary nervous system. 



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