THE JUNCTIONAL TISSUE 55 



from the inter-auricular septum. These converge upon the 

 auriculo- ventricular node, a mass of tissue lying on the right 

 border of the septum in the neighbourhood of the coronary 

 sinus. From this node emerges the auriculo- ventricular 

 bundle, which passes forward, still on the right side, to the 

 central fibrous body of the heart. At the anterior end of the 

 pars membranacea of the interventricular septum, it 

 divides into two branches, the right branch passing imme- 

 diately beneatli the endocardium to the papillary muscles, 

 where it arborises. The left branch, after piercing the pars 

 membranacea, proceeds downwards along the left side of 

 the septum, where it arborises. The extensive arborisations 

 on both sides are known as the Purkinje fibres. These, 

 ramifying in the subendocardial tissue, eventually terminate 

 by becoming continuous with the ventricular substance, 

 and in particular with the papillary muscles. 



It is important to realise that throughout its course 

 the fibres of the junctional system are surrounded by 

 connective tissue which isolates them from the main 

 ventricular mass until their termination is reached. 



At the auriculo- ventricular bundle, the fibres resemble 

 those of the sino-auricular node in their shape, and in their 

 isolation by connective tissue. But as they are traced 

 downwards, the cells come to have a less plexiform, more 

 parallel disposition, they become paler and larger, the 

 nucleus is multiple, and the striation is confined to the 

 periphery of the cells. 



Chemically, the junctional tissue differs from the con- 

 tractile in containing a high percentage of glycogen. 



The auricido-ventricular bundle forms the only connection, 

 other than fibrous, between auricles and ventricles. 



The Function of the Junctional Tissue 



The Sino-auricular Node is the Pace-maker of the Heart 



We have seen that when a wave of contraction passes 

 along a muscle, the part which is in contraction is electro- 



