512 



ANGIOLOGY 





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 inter- 

 medium, and the communication between the atria is reestablished 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. 



Left duct of Cuvier 



Opening of coronary 

 sinus 



Foramen ovale 

 Probe in aorta 

 Aortic septum 



Septum 



intermedium 



Septum 

 inferius 



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



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 fossae 

 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. 

 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 



