Summary. The crista terminalis (CT) with musculi pectinati was isolated from the right atrium: it discharged at a frequency intermediate to that of the 2 nodes. Pacemaker action potentials were recorded from the CT deep layer fibers. The results suggest the presence of a subsidiary atrial pacemaker in the CT deep layer.Previous research has demonstrated that the crista terminalis (CT) is composed of 2 layers of fibers l'2. The deeper fibers differ from the common atrial fibers of the CT superficial layer and musculi pectinati, in that they are clear, large-sized, arranged in parallel and possess Ttubules. They form a homogeneous thick layer which extends to the epicardium for the whole length of the crista. Action potentials recorded from the deep layer fibers have a plateau in the repolarizing phase and a very high maximum rate of rise (Vm,x) and amplitude. These findings led us to attribute to the CT deep layer the role of rapidly conducting excitation to the atrioventricular node (AVN) 2. Because we occasionally recorded action potentials with a slow diastolic depolarization from the CT deep layer and noting, in addition, that the presence of atrial ectopic foci have been localized by extracellular recordings in the dog's heart 3-5, experiments were carried out to determine whether the CT deep layer fibers can act as pacemaker when separated from the sinoatrial node (SAN) and, eventually, from the AVN. Methods. Hearts were excised from 30 rabbits (weighing 2.5-3 kg) anesthetized with pentobarbitone sodium. The isolated right atrium was prepared as described by Paes de Carvalho et al. 6 and subsequently dissected into 3 preparations: 1. the SAN, 2. the CT and musculi pectinati, 3. the AVN. The 3 preparations, maintained at 37+0.3 ~ were perfused with a Tyrode solution, equilibrated with 02 95% and CO 2 5%. The composition of the solution in mM was: NaC1 137, KC1 2.7, CaC12 t.9, NaHCO 3 12, glucose 5.5, MgC12 0.5, NaH2PO4 3.6. The pH of the solution was between 7.2 and 7.4. Further dissection to expose the deep layer of the CT was done carefully by peeling off a thin strip of the superficial layer. Action potentials and Vmax were recorded from the fibers of the 3 preparations with floating glass microelectrodes by conventional methods. In order to verify that the CT was properly isolated and tha! the tissue was well preserved, microscopic and ultrastructural controls were made. The tissue was fixed by substituting a fixing solution for the Tyrode solution, either a Karnowsky diluted solution 7 or the fixing solution referred to by Janse et al. 8. Results and discussion. Following the dissection of the SAN region from the beating right atrium the isolated SAN continued discharging at a mean rate of 156+7/min. The remaining part, after a few seconds' standstill, started to discharge at a mean frequency of 126+ 10/min. Dissection of the AVN from the CT preparation caused standstill of the former, while the CT continued discharging at the same 500 "6 IO0
