278 THE NERVES OF THE HEART. [BOOK i. 



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. 



In the frog (and other amphibia) the arrangement differs 

 somewhat from the above plan and therefore needs a special 

 description. 



The only nerves going to the heart of the frog are the two 

 vagi, right and left, which may be seen running along the two 

 superior venae cavae, and becoming 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 splanchnic or 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 vagi, 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, Bidder's 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 with 

 which are small groups of cells, to the auricular walls, and to the 

 sinus walls, and on the other hand the two main nerves running 

 along the septum, in connection with which are small ganglia 

 which may be called ' septal ' ganglia. From the latter, Bidder's 

 ganglia, fibres unaccompanied except for a short distance by nerve 



