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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The inferior vena cava, larger than the superior, returns the blood from the lower half of the body, and opens into the lowest part of the atrium, near the atrial septum, its orifice being directed upward and backward, and guarded by a rudimentary valve, the valve of the inferior vena cava (Eustachian valve). The blood entering the atrium through the superior vena cava is directed downward and forward, i.e., toward the atrioventricular orifice, while that entering through the inferior vena cava is directed upward and backward, toward the atrial septum. This is the normal direction of the two currents in fetal life.
  The coronary sinus opens into the atrium, between the orifice of the inferior vena cava and the atrioventricular opening. It returns blood from the substance of the heart and is protected by a semicircular valve, the valve of the coronary sinus (valve of Thebesius).


FIG. 493– Interior of right side of heart. (See enlarged image)

  The foramina venarum minimarum (foramina Thebesii) are the orifices of minute veins (venœ cordis minimœ), which return blood directly from the muscular substance of the heart.
  The atrioventricular opening (tricuspid orifice) is the large oval aperture of communication between the atrium and the ventricle; it will be described with the right ventricle.
  The valve of the inferior vena cava (valvula venœ cavœ inferioris [Eustachii]; Eustachian valve) is situated in front of the orifice of the inferior vena cava. It is semilunar in form, its convex margin being attached to the anterior margin of the orifice; its concave margin, which is free, ends in two cornua, of which the left is continuous with the anterior edge of the limbus fossæ ovalis while the right is lost on the wall of the atrium. The valve is formed by a duplicature of the lining membrane of the atrium, containing a few muscular fibers. In the fetus this valve is of large size, and serves to direct the blood from the inferior vena cava, through the foramen ovale, into the left atrium. In the adult it occasionally persists, and may assist in preventing the reflux of blood into the inferior vena cava; more commonly it is small, and may present a cribriform or filamentous appearance; sometimes it is altogether wanting.

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