Abstract
1. In order to study the excitation conduction between cardiac muscle and vascular muscle in venae cavae, the transmembrane action potentials and the unipolar surface electrograms were recorded simultaneously from the atriocaval preparation isolated from a rabbit.
2. The impulse which originates in the sinus node spreads not only to the atrial muscle but also to the vascular muscle in venae cavae (sino-caval conduction).
3. Sino-caval block was frequently observed to occur under the condition where sino-atrial block was not observable.
4. The transmembrane action potential with a hump on the upstroke was recorded from the region of the transition from the fibers of the sinus node to those of the right superior vena cava. After a prolonged period of experiment, the hump became prominent and the spike was frequently extinguished leaving only a small potential.
5. From the mode of appearance of small potentials and notched action potentials, it is suggested that the vena caval margin of the sinus node presents obstacles to smooth passage of excitation and that thus sino-caval conduction is more easily and more markedly depressed than sino-atrial con-duction under some poor conditions.
