HEART 



385 



the primitive tissue — the sino-auricular node — is found at the 

 junction of the superior vena cava and the right auricle, the 

 other — the auriculo-ventricular node — extends from a point 

 just below and to the left of the coronary sinus to the upper 

 part of the inter-ventricular 

 septum to form the aur- 

 iculo - ventricular band. 

 Here it divides into a right 

 and left bundle which pass 

 down under the lining of 

 the heart (the endocardium) 

 to end in line ramifications 

 in the papillary muscles 

 and in the ventricular wall. 

 A continuation of the primi- 

 tive tissue, from the former 

 to the latter of these nodes, 

 along the posterior walls of 

 the auricles, has been de- 

 scribed. 



The essential features of 

 cardiac muscle are (i) that the fibres are continuous with 

 one another, and form a syncytial network ; (ii) that the 

 network contains fibrils which are cross striped (p. 205) ; 

 (iii) that there are nuclei placed deeply in the sarcoplasm ; 

 and (iv) that a sarcolemma is absent. 



In the nodal tissue and auriculo-ventricular band, the 

 fibres have a larger quantity of sarcoplasm between the 

 fibrils and round the nuclei. 



The muscular fibres of auricles and ventricles take origin 

 from three fibrous rings — the auriculo-ventricular rings — 

 (1) one encircling the opening between the right auricle and 

 ventricle, and crescentic in shape ; (2) one, more circular in 

 shape, encircling in common the left auriculo-ventricular and 

 the aortic orifice, and (3) one encircling the pulmonary 

 opening. The auricles are attached to the auriculo- 

 ventricular rings above, the ventricles are attached below, 

 while the valves of the heart are also connected with 

 them. 



25 



Fig. 163. — To show the Development of 

 Auricle, Ventricle, and Bulbus on the 

 Primitive Tube of the Heart. 



