314 THE HUMAN BODY 



sinus with the auricles and of the auricles with the ventricles. 

 Anatomical study shows that at these junctions most of the 

 cardiac tissue proper is replaced by connective tissue, so that 

 physiological communication between one chamber and another 

 is restricted to small bundles of conducting heart tissue. The de- 

 lay at the junctions is usually explained as resulting from the 

 small size of these conducting paths, which offer on that account 

 considerable resistance to the passage of the contraction wave. 



Neurogenic and Myrogenic Theories of the Heart Beat. There 

 are two questions of fundamental importance to an understanding 

 of the mechanism of the heart's action. These are: (1) Does the 

 rhythmic property of the heart reside in its muscular elements 

 or in its nervous elements? and (2) Is the contraction wave con- 

 ducted over the heart by muscle or by nerve-tissue? By the 

 early students of the heart both these properties were attributed 

 to its nervous elements as being more like nerve activities in gen- 

 eral than like those of muscle; and also because the venous end of 

 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 



