HUMAN PHYSIOLOGY 



contractions, although usually the sinus initiates it. The 

 sinus more than any other part of the heart has the 

 property of rhythmic contraction (Practical Physiology, 

 Chap. IX.). 



We have no evidence that the nerve cells in the sinus or 

 elsewhere have anything to do with this ; and so far as we 

 at present know, the initiation as well as the propagation of 

 the cardiac contraction is a function of the muscular fibres. 



3rd. Intra-cardiac Nervous Mechanism. In the frog's 

 heart nervous structures exist, and are distributed as 

 follows : 



1st. In the wall of the sinus venosus there are a number of 



AOFTTA 



V.C.I. 



FlQ. 117. Scheme of the various chambers of the Frog's Heart and of the 

 distribution of the intracardiac nervous mechanism. 



nerve cells constituting the ganglion of the sinus (Remak's 

 ganglion). 



2nd. In the inter-auricular septum a number of nerve cells 

 constitute the ganglion of the auricular septum. 



3rd. In the auriculo-ventricular groove a number of 

 nerve cells are also found forming the auriculo-ventricular 

 ganglion (Bidder's ganglion). With these intra-cardiac 

 ganglia the terminations of the vagi nerves form definite 

 synapses. 



In the mammalian heart nerve cells exist, but there is not 



