280 MANUAL OF PHYSIOLOGY. 



cess may, therefore, with profit be studied in a cold-blooded 

 animal, where the mechanisms can be more readily observed, and 

 are'probably more simple in arrangement. 



The frog, being readily obtainable, is commonly chosen. 



If the apex of the ventricle of the frog's heart be separated, 

 it remains motionless, while the auricles continue to beat. But 

 it responds to short direct stimulus by an ordinary single con- 

 traction, and if the stimulus be kept up it beats rhythmically. 

 If the auricles be removed from the ventricles so as to leave 

 the line of union attached to the ventricle, both continue to 

 beat. But each part beats with a different rhythm, and under 

 like conditions the auricles continue to beat longer than the 

 ventricles. 



The auricles beat even when subdivided ; and the dilated ter- 

 mination of the great vein, called the sinus venosus, opening into 

 the right auricle, when quite separated from the rest of the heart, 

 continues to beat longer and more regularly than any other part. 

 When the entire heart is intact, this sinus seems to be the starting- 

 point of the heart beat. 



This experimental evidence of the presence of nerve centres in 

 the heart muscle is supported by the results of anatomical inves- 

 tigations, for the microscope shows that there are many ganglion 

 cells distributed throughout the heart tissue, and that they are 

 located just where we should expect from the above facts. That 

 is to say, there are none in the substance of the ventricles, while 

 there are several groups of cells scattered around its base in the 

 auriculo-ventricular groove (Bidder). There are others in the 

 walls of the auricles, particularly in the septum, and the greatest 

 number are found in the walls of the sinus venosus (Remak). 



The ganglia in the sinus venosus are most easily stimulated, 

 and are probably the only ones which habitually act as automatic 

 centres. They certainly take the initiative in the ordinary heart 

 beat, and regulate the rhythm of the contraction of the auricles 

 and ventricles. 



This seems more than probable from the following facts : 1. 

 The ordinary contraction wave starts from the sinus venosus. 2. 

 This part beats longer and more steadily than the others when 



