BIOLOGY 





The truncus arteriosus and bulbus cordis are divided by the aortic septum (Fig. 

 470). This makes its appearance in three portions. (1) Two distal ridge-like 

 thickenings project into the lumen of the tube; these increase in size, and ultimately 

 meet and fuse to form a septum, which takes a spiral course toward the proximal 

 end of the truncus arteriosus. It divides the distal part of the truncus into two 

 vessels, the aorta and pulmonary artery, which lie side by side above, but near 

 the heart the pulmonary artery is in front of the aorta. (2) Four endocardial 

 cushions appear in the proximal part of the truncus arteriosus in the region of the 

 future semilunar valves; the manner in which these are related to the aortic septum 

 is described below. (3) Two endocardial thickenings anterior and posterior 



Second aortic arch 

 Third aortic arch 



First aortic arch 



Auditory vesicle 



Primitive jugular vein 

 Fourth aortic arch 



Sixth aortic arch 

 Dorsal aorta 



Cardinal vein 



Digestive tube 



Maxillary process 

 First branchial groc 

 Mandibular arch 



Bulbus cordis 

 A trimn 

 Duct of Cuvier 

 Ventricle 



Hind-gut 



Umbilical vein 





Olfactory pit 



Allantois 

 Umbilical artery 



FIG. 472. Profile view of a human embryo estimated at twenty or twenty-one days old. (After His.) 



develop in the bulbus cordis and unite to form a short septum; this joins above with 

 the aortic septum and below with the ventricular septum. The septum grows down 

 into the ventricle as an oblique partition, which ultimately blends with the ven- 

 tricular septum in such a way as to bring the bulbus cordis into communication 

 with the pulmonary artery, and through the latter with the sixth pair of aortic 

 arches; while the left ventricle is brought into continuity with the aorta, which 

 communicates with the remaining aortic arches. 



The Valves of the Heart. The atrioventricular valves are developed in relation to 

 the atrial canal. By the upward expansion of the bases of the ventricles the canal 

 becomes invaginated into the ventricular cavities. The invaginated margin forms 

 the rudiments of the lateral cusps of the atrioventricular valves; the mesial or 



