HEART 



389 



(6) In the inter-auricular septum a similar plexus consti- 

 tutes the ganglion of the auricular septum. 



(c) In the auriculo-ventricular groove a plexus forms 

 the auriculo-ventricular ganglion (Bidder's ganglion). 



With these intra-cardiac ganglia the terminations of the 

 cardiac branch of the vagus nerve to the heart form definite 

 synapses. 



(2) In the mammalian heart, on the right side an 

 interrupted chain of nerve cells extends from near the sino- 

 auricular node to the posterior part of the inter-auricular 

 septum and down to the auriculo-ventricular node. On the 



Fig. 166. — Scheme of the Various Chambers of the Frog's Heart and of the 

 Distribution of the Intracardiac Nervous Mechanism. 



left side, a similar chain of cells begins rather lower on the 

 back of the left auricle and extends inwards to the septum 

 to end near the auriculo-ventricular node. Through all the 

 primitive tissue there is a dense plexus of nerve fibres, and 

 this, according to Dogiel and Bethe, also extends between 

 the ordinary fibres of auricles and ventricles. Bodies like 

 muscle spindles have been described among the fibres. 



3. Pericardium. — The myocardium is covered by a layer 

 of fibrous tissue with endothelium on its surface. This is 

 the visceral pericardium. The parietal pericardium, in which 

 the heart lies, is a fibrous sac lined by endothelium, firmly 



