The origin of the cardiac impulse in the turtle's heart

Recent work has emphasized the sinoauricular node as the seat of origin of the cardiac impulse in the normal heart. This structure must be considered anatomically as forming a connection or junction between remains of the primitive sinus, which has disappeared as a separate chamber in the mammalia, and the auricle. Since indications of the presence of a sinoauricular junction, composed of tissue differentiated histologically from the ordinary cardiac muscle, is found not only in those hearts in which the sinus has disappeared as a separate chamber, but also in the amphibia and reptiles, where a separate sinus venosus is present, it becomes of essential importance to determine whether or not in these animals the heart beat arises as it does in the mammalian heart, that is in the sinoauricular junction. This problem may be attacked by determining that region which shows initial electric negativity when connected with the string galvanometer. We have had this problem in mind for some time, but have been unable to proceed with it because of the difficulty in securing material with sufficiently large hearts to test this point satisfactorily. Recently we have been able to secure one large turtle in which the heart was of sufficient size. In this the right half of the sinus was 35 millimeters long and 20 millimeters wide at its junction with the left half. We compared the onset of negativity at the sinoauricular junction with the right and left halves of the sinus, and found the junction to precede in negativity. It must be borne in mind, however, that in the exposed heart, automaticity and conductivity may sometimes be abnormal, and a fairly long series of observations will be necessary in order to determine without question, the normal origin of the impulse.