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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
transverse slit, and two thickenings appear, one on its dorsal and another on its ventral wall. These thickenings, or endocardial cushions (Fig. 465) as they are termed, project into the canal, and, meeting in the middle line, unite to form the septum intermedium which divides the canal into two channels, the future right and left atrioventricular orifices.
  The primitive atrium grows rapidly and partially encircles the bulbus cordis; the groove against which the bulbus cordis lies is the first indication of a division into right and left atria. The cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum (Fig. 465), which grows downward into the cavity. For a time the atria communicate with each other by an opening, the ostium primum of Born, below the free margin of the septum. This opening is closed by the union of the septum primum with the septum intermedium, and the communication between the atria is reëstablished through an opening which is developed in the upper part of the septum primum; this opening is known as the foramen ovale (ostium secundum of Born) and persists until birth. A second septum, the septum secundum (Figs. 467, 468), semilunar in shape, grows downward from the upper wall of the atrium immediately to the right of the primary septum and foramen ovale. Shortly after birth it fuses with the primary septum, and by this means the foramen ovale is closed, but sometimes the fusion is incomplete and the upper part of the foramen remains patent. The limbus fossæ ovalis denotes the free margin of the septum secundum. Issuing from each lung is a pair of pulmonary veins; each pair unites to form a single vessel, and these in turn join in a common trunk which opens into the left atrium. Subsequently the common trunk and the two vessels forming it expand and form the vestibule or greater part of the atrium, the expansion reaching as far as the openings of the four vessels, so that in the adult all four veins open separately into the left atrium.


FIG. 468– Same heart as in Fig. 467, opened on right side. (From model by His.) (See enlarged image)

  The primitive ventricle becomes divided by a septum, the septum inferius or ventricular septum (Figs. 465, 466, 467), which grows upward from the lower part of the ventricle, its position being indicated on the surface of the heart by a furrow. Its dorsal part increases more rapidly than its ventral portion, and fuses with the dorsal part of the septum intermedium. For a time an interventricular foramen

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