INHIBITION AND CARDIAC VAGUS 111 



nerves leave the cranial system, it seems as if the actual 

 facts had been obscured by hasty theories of inhibition, 

 and that the portion of the bulb where the vagus arises is 

 the actual centre which is still looked for. We might, then, 

 assume that vagus action is the same as positive vaso- 

 dilatation, however much the phenomena are obscured in the 

 intestine or elsewhere by subsidiary controlling mechanisms, 

 such as are probably found in Auerbach's plexus, and 

 infer that the whole action of the heart is but specialized 

 vaso-constriction and dilatation by an ancestral motor 

 nerve and its later subordinate ganglia. It might even 

 be said that the Keith-Flack node is the Auerbach plexus 

 of the heart. If this is so, vaso-dilatation is everywhere 

 caused by a positive elongation of a muscular ring, which 

 pushes outwards while held in position by neighbouring 

 tissues ; the cell-lengthening being caused by osmosis. 

 When we deal, not with such small muscular systems as an 

 intestinal or arterial coat, but with a larger connected 

 mass of systems such as cardiac muscle, it is not more 

 difficult, or so it seems to me, to understand how a negative 

 pressure comes about in the ventricle than in a pump. In 

 fact, the heart in more senses than one is a double pump, for 

 it not only expels blood but draws it in. On further experi- 

 ment it may be found that even the auricles have a feeble 

 aspiratory power. I find myself totally unable to credit 

 any other view since the heart, when removed from an 

 animal and kept in a nutrient Ringer's solution, con- 

 tinues to expand and contract actively. Very many 

 years ago I was much impressed by observing the heart of 

 a pelagic shark, of the genus Carcharias, beating in the 

 open air of a hot tropical day. I held it in my hand, and 

 found some pressure needed to keep it closed. On opening 

 my fingers it followed them, and went on beating. I put 



