THEORIES OF THE .HEART-BEAT 203 



is not due to the nerves of the heart, say the myogenists, but to the 

 inherent property of the muscle itself. 



It was thought for a long time that in the mammal there was no mus- 

 cular continuity between the auricles and ventricles to conduct the con- 

 traction wave and that this was an insurmountable difficulty in the way of 

 accepting the myogenic theory of the heart beat. In 1893 Kent described 

 a bundle of muscle fibers arising in the wall of the right auricle and near the 

 septum and running down into and forming a muscular connection with the 

 ventricles. This bundle was also independently described by His, Jr., 

 and generally bears his name. This band is called the auriculo- ventricu- 

 lar bundle. 



It is now generally recognized that the early embryonic cardiac tissue 

 undergoes differentiation in two directions. Out of one of these types 

 of tissue there is produced the wellknown cardiac muscular tissue which 

 makes up the mass of the auricles and the ventricles. Out of the other 

 differentiation is produced the type of tissue which constitutes the auri- 

 culo-ventricular conducting or bundle system. This conducting type 

 of tissue is striated like the ordinary cardiac tissue but in general appear- 

 ance is more embryonic in type. Its cells constitute what is known as the 

 Purkinje fibers. The main bundle described by His, Jr., runs in the inter- 

 ventricular septum somewhat lightly buried in the tissue beginning at the 

 base of the auricle on the right side and running down through the inter- 

 auricular tissue to the septum of the ventricles where it divides into a 

 right and left branch. Strands of this tissue extend somewhat up into 

 the auricles but are elaborately developed into a net work lying just under- 

 neath the endothelium of the right and left ventricles. The branch- 

 ing net work of these cells shades into and is continuous with cells of the 

 ordinary cardiac type. Miss De Witt (1909) made an excellent model 

 of this system which has become classic in the literature and is reproduced 

 in figure 106. The bundle system contains two regions known as nodes, 

 the sino-auricular node imbedded in the wall of the right auricle just in 

 the angle where it is joined by the superior vena cava; and the auriculo- 

 ventricular node, described first by Tawara, which lies in the upper end of 

 the His bundle. The physiological differentiation of this tissue is in the 

 direction of rhythm production and facilitated conduction. 



The demonstration of the auriculoventricular bundle has proven to be 

 of the strongest support to the myogenic theory. Erlanger has shown, 

 by an ingenious device for partially clamping this muscular band, that 

 even the mammalian ventricle exhibits the phenomenon of heart block. 

 The sequence of auricle and ventricle can be perfectly controlled by the 

 degree of compression exerted by the clamp. In his experiments the 

 ventricle contracts in unison with every auricular contraction, or only 

 every second or every third, according to the degree of blocking. 



