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 obta ined from two cadavers were histologically examined with regard to the distribution of cardiac muscle fibers in their walls. The superior ve na cava contained cardiac muscle fibers together with smooth muscle fi bers. The cardiac muscle fibers mere 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 i n bundles longitudinally, obliquely, or circularly. Cardiac myocytes o ccupied one to two thirds of the wall thickness, but decreased in amou nt toward the periphery. The inferior vena cava also contained both ca rdiac and smooth muscle fibers. The cardiac muscle fibers extended con tinuously, covering a distance of 18 mm from the atrium to a level jus t under the diaphragm. Their fibers were bundled, running circularly o r obliquely, and being more abundant in the anterior wall than in the posterior. From these findings, the venae cavae close to the atrium ar e histologically regarded as an extension of the atrium. The venae cav ae close to the atrium probably contract together with the atrium and work as a functional valve that contributes to the pumping mechanism o f the heart.