LATER DEVELOPMENT OF VASCULAR SYSTEM 307 
ridges, and that the aortic division of the bulbus lies above the 
pulmonary division, that is, next the bulbo-auricular ridge. 
The left bulbus ridge is thus continuous with the interventricular 
septum immediately beneath the foramen of the latter, and the 
right bulbus ridge lies on the opposite side. 
The bulbus septum now becomes complete by fusion of the 
right and left sides. The blood from the left ventricle is then 
forced in each systcle through the interventricular foramen and 
along a groove in the right side of the cushion-septum into the 
aortic trunk. This groove, however, is open to the right ven- 
tricle also above the septum of the bulbus; but it is soon bridged 
over by an extension of the cushion-septum along the bulbo- 
auricular ridge as far as the right side of the septum of the bulbus; 
in this way the space existing between the interventricular sep- 
tum and the opening of the aorta is converted into a tube, and 
thus the aorta is prolonged through the cushion-septum, and 
by way of the interventricular foramen into the left ventricle. 
Fate of the Bulbus. The distal portion of the bulbus is con- 
verted into the proximal parts of the aorta and pulmonary artery. 
The part proximal to the semilunar valves is gradually incor- 
porated into the ventricles, owing to extension of the ventricular 
cavities into its wall, and subsequent disappearance of the inner 
wall of the undermined part. 
The Sinus Venosus. (lor earlier development see Chap. VI; 
relation to septum transversum, Chap. XI.) 
In the course of development, the sinus venosus gradually 
separates from the septum transversum, though always connected 
with the latter by the vena cava inferior. In early stages (up to 
about 24 somites) it is placed quite symmetrically behind the 
atrium, and extends transversely to the entrance of the ducts of 
Cuvier on each side. The sinu-auricular aperture is approximately 
in the median line at first, so that the right and left divisions of 
the sinus are nearly symmetrical. The condition of approximate 
bilateral symmetry of the sinus is, however, rapidly changed 
by shifting of the sinu-auricular aperture to the right side with 
the outgrowth of the right auricle (24-36 somites); thus the left 
horn of the sinus becomes elongated; moreover, the main expan- 
sion of the sinus takes place in the region of the sinu-auricular 
aperture, and thus the left horn appears relatively narrow in diam- 
eter. The interauricular septum forms to the left of the sinu- 
