350 THE HUMAN BODY 



the heart, where the beat originates, contains more nervous matter 

 than do the other chambers. More recently the view that both 

 rhythmicity and conductivity are cardinal functions of the heart's 

 musculature began to receive considerable attention, chiefly 

 through such observations as that the apex of the ventricle, which 

 is devoid of nerve-cells, may be made to show true rhythmicity, 

 and that a series of zigzag cuts, sufficient to sever all direct nerve 

 paths although leaving ample muscular connections, can be made 

 in the ventricle without preventing the passage of the contraction 

 wave over it. With recognition of the probability that the nervous 

 elements of the heart form, not a synaptic system with irreversible 

 conduction, but an intercommunicating plexus which may con- 

 duct in all directions, most of the evidence in favor of the myogenic 

 theory seems less conclusive than it did at first, so that the prob- 

 lems of which is the rhythmic and conducting tissue, or whether 

 both properties are possessed by both tissues, are still far from 

 settled. 



The Nature of Automatic Rhythmicity. It should be clearly 

 understood that the question whether rhythmicity is a property of 

 cardiac muscle or of cardiac nerve-tissue is quite distinct from the 

 question of the underlying nature of rhythmicity itself. Much 

 study has been given to this latter problem and here again two 

 opposing views are held. One of these is that the heart is sub- 

 ject to the influence of a constant stimulus, its property of "maxi- 

 mal" contractions with their accompanying refractory periods 

 sufficing to bring about rhythmic responses to such constant 

 stimulation. The other view is that the heart is a truly automatic 

 organ, the metabolic processes going on within the heart tissue 

 being of such a nature as to produce rhythmic activity quite 

 independently of "stimulation" as we ordinarily understand it. 



Those who believe the heart to be under the influence of a 

 constant stimulus look to the blood as its source, and especially 

 to the inorganic blood-salts, it having been shown that the heart- 

 beat can be maintained for an astonishing length of time when 

 the heart is fed solutions containing only inorganic salts of sodium, 

 potassium, and calcium in proper proportion. Those who look 

 upon the heart as a truly automatic organ take the position that 

 their view is more in accordance with general physiological prin- 

 ciples than the other, and that no evidence yet brought forth 



