SEQUENCE OF CONTRACTION OF PARTS OF HEART, 181 



cells which pass from the sinus to the auriculo-ventricular junction, so 

 that, as seen in Fig. 105, if the cut be carried far enough, the auricle is 

 divided into two parts, As, a part connected with the sinus, and Av, 

 a part connected with the ventricle, and the only connection still 

 left between sinus and ventricle is by means of a narrow bridge of 

 tissue at the apex of the auricle which connects As and Av. Conversely, 

 owing to the presence of the free coronary nerve between the sinus and 

 auriculo-ventricular junction, it is possible to cut through the whole of 

 the auricle and leave only the coronary nerve as connection between 

 sinus and ventricle. 



These two experiments show conclusively that the sequence of 

 ventricular upon auricular contraction is not dependent upon the 

 integrity of the nerve trunks which pass from the sinus to the auriculo- 

 ventricular groove. For, in the first case, the sequence is never affected 

 until the bridge of tissue at the apex of the auricle has become very 

 small ; and, in the I 



second case, when 

 the coronary nerve 

 only is left, the 

 rhythm of the 

 beats of the ven- 

 tricle and that 

 part of the auricle 

 still in connection 

 with it never bears 

 any relationship 

 to the rhythm of 

 the sinus, and that 

 part of the auricle 

 still left in con- 

 nection with the 

 sinus. 



From these 

 experiments it is 

 seen that the ven- 

 tricle contracts in 

 sequence to the auricle, only when a wave of contraction is able to pass 

 along As and Av. 



I was able further to show that if the piece of tissue between As and 

 Av is reduced sufficiently, then a block to the passage of the con- 

 traction from As to Av takes place of such a character, that, if the 

 block is very slight, a distinct pause occurs at this point when the 

 contraction wave which passes from the sinus along As reaches it 

 before it passes along Av, a pause distinctly comparable to and 

 resembling the pause which takes place at the auriculo-ventricular 

 junction between the contraction of Av and the ventricle. If the block 

 is more severe, then, instead of every contraction passing the blocking 

 point, only every second contraction is able to pass, and therefore a 

 contraction of the ventricle follows upon only every second auricular 

 contraction. With a still more severe block it is possible to see only 

 every third or every fourth contraction able to pass, and therefore to see 

 the ventricle beating to every third or fourth contraction of the auricle. 

 This latter rhythm cannot be obtained for any length of time, but it is 



V 



FIG. 105. Heart of tortoise, suspended as in Fig. 104, to show 

 appearance of preparation when a block is caused by slitting 

 up the auricle. 



