294 THE NERVES OF THE HEART. [BOOK i. 



similar ganglionic collar at the junction of the auricle with the 

 ventricle, where also there is similarly retained a circular dis- 

 position of the muscular fibres forming the so-called canalis auri- 

 cularis. And indeed in all vertebrates two similar collections of 

 ganglia are more or less distinctly present. There are ganglia 

 at the junction of the sinus with the auricle and along the 

 entering nerve branches ; these may be called the sinus ganglia. 

 There are other ganglia at the junction of the auricle and ven- 

 tricle ; these may be called the auriculo-ventricular ganglia. 

 Besides these two groups there are also ganglia over the auricle 

 in connection with nerves passing from the sinus to the ventricle. 



Lastly, as a general rule the main nerve branches and the 

 ganglia are not plunged deep in the substance of the heart, but 

 are placed superficially, immediately under the pericardial layer. 

 From the cells and nerves so situated finer branches and fibres 

 pass to the substance of the heart. 



Bearing this general plan in mind we may now turn to the 

 special arrangements which obtain in the frog and in the 

 mammal. 



In the Frog. The only nerves going to the heart are the two 

 vagus nerves, right and left, which may be seen running along the 

 two superior vena3 cavse, but become lost to view at the sinus where 

 they pass from the surface to deeper parts. Each vagus is not 

 however simply a vagus nerve, but as we shall see contains fibres 

 derived from the sympathetic system. As the nerves approach 

 the sinus, groups of nerve cells become abundant in connection 

 with the fibres, and as the fibres spread out at the sinus many 

 ganglia are scattered among them, forming what is called as a 

 whole the sinus ganglion or the ganglion of Remak. 



From the sinus the two vagus nerves, leaving their position 

 under the pericardium, plunge into the heart and run along the 

 septum between the auricles, on the left side of the septum, one, 

 the anterior nerve, passing nearer the front of the heart than the 

 other, the posterior. Several groups of cells, or small ganglia, 

 are connected with the two ' septal ' nerves' thus passing along the 

 septum. 



The nerves reaching the auriculo- ventricular ring on the an- 

 terior side of the heart end in two ganglia lying at the base of the 

 two large auriculo-ventricular valves. 



From these two ganglia, Bidders ganglia or the auriculo- 

 ventricular ganglia, nerve fibres pass into the substance of the 

 ventricle. Nerve cells may be traced on the fibres going to the 

 ventricle for some little distance, but for a little distance only; 

 over the greater part of the ventricle, the lower two-thirds for 

 instance, the nerve fibres are free from nerve cells. 



Thus in the frog there are two main ganglia, sinus or Remak's 

 ganglion, auriculo-ventricular or Bidder's ganglia. From the 

 former there pass on the one hand scattered fibres, in connection 



