526 



THE BLOOD-VASCULAR SYSTEM 



The left horn and transverse part, now only draining the dwindling left common cardinal, (left 

 superior cava) and the coronary veins, become the coronary sinus. The right horn gradually 

 becomes absorbed into the right end of the atrial cavity until the superior and inferior cava? 

 and the coronary sinus acquire separate openings into that chamber. Between the opening 

 of the coronary sinus and that of the inferior cava there is a ridge, the sinus-septum (between 

 the right horn and transverse parts of the sinus), which becomes attached to the lower part 

 of the right sinus valve. 



In the atrium a septum begins early to grow from the ventro-cephahc wall of the atrium, 

 toward the atrial canal. As the interatrial communication around the edge of the septum 

 (ostium primum) is becoming narrow, a perforation occurs near the attached margin of the 

 septum (ostium secundum). This first septum (septum primum) is incomplete because when 

 its edge reaches the atrial canal the atria still communicate through ostium secundum. To the 

 right of the septum primum another septum (s. secundum) is formed later; this never stretches 

 completely across the atrium and is rather a crescentic ridge than a true septum. Until the 

 free edges of the two septa overlap one another there is a direct passage leading from one side 

 of the atrium to the other; eventually they do overlap and the communication becomes 

 oblique but persists until birth. (For adult relations of septa, see p. 511.) The cavities 

 resulting from the division of the common atrium are the right and left atria of the adult. 

 The oblique channel connecting the atria (foramen ovale) is bounded on the right side by the 

 s. secundum the free edge of which forms the limbus fossae ovalis. The channel is bounded 

 on the left by the s. primum which slants into the left atrium. The free edge of the s. 

 primum becomes the valvula foram.inis ovalis; the remainder, the membranous atrial septum of 

 the adult. 



Fig. 440. — Reconstrtjction of the Heart of an 11 mm. Human Embryo viewed from 



Below. (Mall, 50.) 

 The lower part of the ventricular portion has been cut off. Connective tissue septa colored 

 yellow. Ao, aorta; Ap, anterior papillary muscle; La, left atrium; Lo, left venous ostium; Lp. 

 large (anterior) papillary muscle of right ventricle; Mpm, medial papillary muscle; PP, pos- 

 terior papillary muscle; P, pulmonary artery; RA, right atrium. 



) 



The portion of the dorsal wall of the right atrium immediately adjoining the septa is derived 

 from the sinus venosus. This part of the atrium (the sinus venarum) receives the great venous 

 openings. The left side of the left sinus-valve is attached to both septa and assists the septum 

 secundum in the formation of the limbus foraminis ovalis. The cephahc part of the right sinus- 

 valve disappears along the line of the (adult) crista terminalis, which therefore limits the right 

 portion of the right atrium derived from the sinus venosus. The caudal portion of this valve 

 persists as the inferior caval and coronary valves. These are drawn out of their original align- 

 ment by the adhesion between the caudal part of the right siiuis-valve and the sinus-septum. 



The left atrium receives, through the dorsal mesocardiuin, the originally single pulmonary 

 vein. Thi.s common stem is absorbed into the atrial wall; later, the i)rimitive right and left 

 tributaries are absorbnd in a similar wav, leaving the four pulmonary veins of the adult open- 

 ing .separately into the left atrium. The area of the left atrium adjacent to the pulmonary 

 veins, therefore, is not part of the original atrial wall. 



Tlie ventricles are divided by a septum {s. niuscularc venlriculnriwi) growing from the caudal 

 wall of the (lotnrrKjn ventricular cavity toward the atrial canal. The canal moves to the right, 

 anrl the dorsal i)art of the; septum blends with the dorsal lip of tlio canal. The free ventral 

 edge of the interventricular s(ii)tum helps to bound the foramen through which blood from the left 

 ventricle must enter the right on its way to the conus arteriosus. The foramen persists until 

 (the free margin of the interventricular septum having been joined by the aortic septum) it 

 becomes the circumference of tlie aortic ostium. 



