1914-] MECHANISM OF THE HEART-BEAT. 299 



are describing. More exaggerated changes still are observed when 

 there is no alteration, either in the size or in the position of the 

 heart ; and these are due to the manner in which impulses are prop- 

 agated to the ventricles from the contracting auricles. The pathway 

 followed normally has already been described, but now the normal 

 path cannot be taken, for it has been partly destroyed. It has been 

 shown that when the conduction bundle to the right ventricle is 



pR T 



^ 



w^J^\^.A^./^ 



Fig. 6. Electrocardiogram from a dog. Leads from oesophagus and 

 anus. Above: first portion is a control; the second is a curve taken after 

 the left branch of the A-V bundle has been cut. Below : control and a curve 

 taken after the right branch of the A-V bundle has been cut. After Roth- 

 berger and Eppinger. 



severed, allowing impulses to reach only the left ventricle, the electro- 

 cardiogram immediately takes on another shape, the shape being an 

 exaggeration of what occurs during enlargements of the left side of 

 the heart. The first ventricular wave is sharp downward and the 

 second an upward deflection. If, on the other hand, a similar injury 

 is done to the conduction bundle to the right ventricle, there is a re- 

 versal of electrocardiographic curve. It consists of a sharp up- 

 ward, followed by a downward deflection (Fig 6). Finally, if a 

 curve of one or other form has been obtained by cutting one or other 

 branch of the conducting bundle, one can, by severing the still unin- 

 jured branch, obtain an electrocardiogram which differs from both 

 the preceding and resembles, though not exactly, the original curve. 

 These changes depend, then, upon the way impulses pass through 



