HEART-BLOCK 313 



septum. It lies in that part of the septum which is covered 

 by the posterior, or septal, cusp of the mitral valve, and, 

 finally, divides into two parts, one for each ventricle. 

 The exact destination of the terminal fibres is relatively 

 unimportant. 



Under normal conditions the rhythmical contractions of 

 the heart begin at the sino-atrial node, and, during atrial 

 systole, they reach the atrio-ventricular node. From there 

 the stimulus is conveyed along the atrio-ventricular bundle 

 and causes ventricular systole. Each atrial systole is therefore 



FIG. 112. Tracings from a case of Complete Heart-Block. 



The waves, a, correspond to the atrial contractions ; the waves, c, correspond to the 

 ventricular contractions, as shown by the tracing from the radial pulse. 



followed immediately by a ventricular systole, so long as the 

 conducting medium is able to perform its duty efficiently. 

 Pathological lesions of the atrio-ventricular bundle may 

 diminish its conductivity, and as a result each atrial systole is 

 not immediately followed by a ventricular systole. In these 

 cases, however, a summation of stimuli produces the effect 

 which a single stimulus is insufficient to produce, and a ven- 

 tricular systole follows every second or every third atrial 

 contraction. This condition is known as Heart-block. A 

 small lesion, involving the atrio-ventricular bundle before it 

 divides, may cause complete heart-block, but a much larger 

 lesion, involving one of its divisions, may give rise only to 



