INNERVATION OF THE HEART. 273 



found to continue to beat for four and seven minutes respectively 

 after the spinal cord and the medulla had been separated. 



These facts prove conclusively that the stimulus which causes 

 the heart to beat rhythmically arises in the muscle tissue of the 

 organ or in close relation to it. Upon physiological grounds 

 alone we might conclude that in the heart tissue of the vertebrata 

 there exist the nerve elements with which we are familiar ana- 

 tomically. These nerve cells only require their nutrition to be 

 kept up by a continued blood supply in order to develop the 

 energy necessary for their function. 



Such collections of nerve elements are called automatic centres, 

 and are made up, like all other origins of nerve force, of gangli- 

 onic cells. 



The heart of mammalian animals so soon ceases to beat, that 

 it forms an unsatisfactory subject for experimental inquiry. The 

 heart's inuervation which will be seen to be a complicated pro- 

 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 contraction, 

 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 witli 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- 



23 



