THE JUNCTIONAL TISSUE 53 



This differentiation in function is associated with a 

 differentiation in structure, the rhythmic fibres retaining 

 their embryonic form and circular disposition, the con- 

 tractile fibres undergoing an approximation to the skeletal 

 form in developing a partial cross-striation. 



As this type of heart develops into the mammahan form, 

 the sinus and auricular canal become lost as separate 

 chambers, and their tissues are submerged by the great 

 hypertrophy of the auricle and ventricle. But they do not 

 disappear. They persist, retaining their embryonic nature, 

 and forming the following structures — 



Developed from the Sinus. 



1. The sino-auricular node.i 



2. Part of the interauricular septum. 



3. The opening of the coronary sinus. , 



Developed from the Sino-auricular Canal. 



4. The auriculo-ventricular junctional tissue, con- 

 sisting of — 



(a) Fibres from the auricular septum to 



(b) The auriculo-ventricular node. 



(c) The auriculo-ventricular bundle (Bundle of 



His) and its two branches. 

 {d) The fibres of Purkinje. 



So important to the modern conception of the heart- 

 beat has been the discovery of these remnants that it is 

 necessary to describe their anatomical disposition, and to 

 show how they differ structurally from the ordinary heart- 

 muscle. 



Disposition and Structure of the Junctional Tissue 



The ordinary cardiac muscle is composed of columns of 

 short cyhndrical fibres, united irregularly to those of 



^ The sino-auricular node of the mammalian heart must not be 

 confused with the sino-auricular canal of the primitive vertebrate 

 heart. 



