252 VETEKINARY PHYSIOLOGY 



If a ligature be tightly applied between the sinus and 

 auricles in the frog (Stannius' Experiment), the sinus continues 

 to beat, and the auricles and ventricle mually stop beating 

 for a longer or shorter period. But ultimately they begin 

 to beat again. Hence it would seem that it is not any 

 special mechanism in the sinus which is essential in starting 

 cardiac contraction. A ligature subsequently applied between 

 auricles and ventricle sometimes starts the ventricle, sometimes 

 the auricles, sometimes neither. Hence we see that any 

 part of the heart has the power of originating rhythmical 

 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.} 



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 propaga- 

 tion of the cardiac contraction is a function of the muscular 

 fibres. 



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

 heart nervous structures exist, and are distributed as 

 follows (fig. 119): 



1st. In the wall of the sinus venosus there is a plexus of nerve 

 cells and nerve fibres constituting the ganglion of the sinus 

 (Kemak's ganglion). 



2nd. In the inter-auricular septum a similar plexus 

 constitutes the ganglion of the auricular septum. 



3rd. In the auriculo-ventricular groove a plexus forms 

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

 these intra-cardiac ganglia the terminations of the nerves 

 to the heart form definite synapses. 



Nerve cells exist in the mammalian heart, but there is not 

 the same differentiation into distinct groups. Nevertheless 

 they are abundant round the mouths of the great veins, 

 round the edges of the inter-auricular septum, and round 

 the auriculo-veutricular groove. 



While there is no evidence that the nervous structures 

 play an important part in starting or keeping up the con- 

 tractions, there is evidence that they exercise a checking or 

 controlling action. 



