A histological study of the cardiac muscle of the human superior and inferior venae cavae.

Human superior and inferior venae cavae at the orifices to hearts obtained from two cadavers were histologically examined with regard to the distribution of cardiac muscle fibers in their walls. The superior vena cava contained cardiac muscle fibers together with smooth muscle fibers. The cardiac muscle fibers were distributed uninterruptedly from the atrium to the root of the azygos vein, covering a length of 45 mm. Cardiac myocytes were present outside the smooth muscle and coursed in bundles longitudinally, obliquely, or circularly. Cardiac myocytes occupied one to two thirds of the wall thickness, but decreased in amount toward the periphery. The inferior vena cava also contained both cardiac and smooth muscle fibers. The cardiac muscle fibers extended continuously, covering a distance of 18 mm from the atrium to a level just under the diaphragm. Their fibers were bundled, running circularly or obliquely, and being more abundant in the anterior wall than in the posterior. From these findings, the venae cavae close to the atrium are histologically regarded as an extension of the atrium. The venae cavae close to the atrium probably contract together with the atrium and work as a functional valve that contributes to the pumping mechanism of the heart.