THE EMBRYO. 



1191 



grows upward from the lower part of the ventricle, its position being indicated on 

 the surface of the heart by a furrow. It extends upward almost as far as the 

 auricular canal but for some time an interventricular foramen exists between it and 

 the septum of the auricular canal (Fig. 748). 



Aortic, septum. 



Common auriculo- 

 , ventricular orifice. 



Pulmonary 



artery. 



Right 



auriculo- 



ventriciilar 



orifice. 



Aortic septum. 



/Aorta. 



Right 

 ventricle. 



Septum Left 

 inferius. ventricle. 



Left 

 auriculo- 



ri-ntriciilar 

 orifice. 



Right 

 ventricle. 



Left 

 Fepttim inferius. ventricle. 



FIG. 748. Showing the development of the septum of the aortic bulb and of the ventricles. (Born.) 



The aortic bulb is divided by the aortic septum. This makes its appearance 

 at _ the distal end of the bulb as two ridge-like thickenings of its .endothelial 

 lining; these increase in size, and, projecting into the lumen, ultimately fuse 

 to form the septum, and thus the aortic bulb is divided into the pulmonary artery 

 and the aorta. The aortic septum takes a spiral course toward the proximal end 

 of the bulb, so that the two vessels lie side by side above ; but near the heart the 

 pulmonary artery is in front of the aorta (Fig. 749). The septum grows down 

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



Aorta. 



Aorta. 



Pulmonary 

 artery. 



Aorta. 



'Pulmo- 

 nary artery. 



Pulmonary 

 artery. 



PIG. 749. Transverse sections through the aortic bulb to show the growth of the aortic septum. The lowest 

 section is on the left, the highest on the right, of the figure. (After His.) 



inferius of the ventricles in such a way as to bring the left ventricle into commu- 

 nication with the aorta and the right with the pulmonary artery. 



Peculiarities of the Foetal Heart. In early foetal life the heart' is placed directly 

 under the head and is relatively of large size. Later it assumes its position in 

 the thorax but lies at first in the middle line ; toward the end of pregnancy it 

 gradually becomes oblique. Its auricular portion is at first larger than the ven- 

 tricular part, and the two auricles communicate freely through the foramen ovale. 

 In consequence of the communication, through the ductus arteriosus, between the 

 pulmonary artery and the aorta, the contents of the right ventricle are mainly 

 carried into the latter vessel instead of to the lungs, and hence the wall of the 

 right ventricle is as thick as that of the left. At the end of foetal life, however, 

 the left ventricle is thicker than the right, a difference which becomes more and 

 more emphasized after birth. 



Further Development of the Arteries. In the vitelline circulation, two arteries 

 were described as coming off from the primitive heart, and running down in front 

 of the developing vertebrae. The first change consists in the fusion of these arteries 

 into one vessel at some distance from the heart; this vessel is the descending 

 thoracic and abdominal aorta. In consequence of the lengthening of the neck the 

 heart falls backward to its lower part and then into the thorax, and the two 

 original arteries, proceeding from the heart to their point of fusion in the common 



