The extent of cardiac muscle in the great veins of the dog

Abstract The extent and arrangement of cardiac muscle in the great veins of adult dogs of both sexes was investigated. Cardiac muscle was found for varying distances in the venae cavae and pulmonary and azygos veins in all animals. The average distances which the muscle extended into these vessels in the beagles were: 1.88, 0.859, 0.525, and 0.622 cm. Similar measurements in the poodle were: 1.35, 0.705, 0.764, and 0.690 cm and in the shepherd: 3.05, 1.36, 1.22, and 0.926 cm. Both the venae cavae and pulmonary veins contained circular, spiral and longitudinal muscle bundles. The cardiac muscle was continuous from the superior venae cavae into the wall of the azygos vein and appeared to occupy the medial layer of the wall in all instances. Elastic and white fibrous connective tissue extended between the muscle bundles and appeared to invest the tapered muscle fibers in some places.