DEVELOPMENT OF THE VASCULAR SYSTEM. 



803 



Fig. 313. 



HEART OF INFANT, showing 

 the mode of disappearance of the 

 arterial duct after birth. 1. Aorta. 

 2. Pulmonary artery. 3, 3. Pulmo- 

 nary branches. 4. Ductus arterio- 

 fcus becoming obliterated. 



pulmonary trunk (4) which is beyond the origin of the pulmonary 

 branches, and which communicates freely with the aorta, is the Ductus 

 arteriosus. 



The ductus arteriosus is at first as large as the pulmonary trunk 

 itself; and nearly the whole of the blood coming from the right ven- 

 tricle, passes through the arterial duct, 

 and enters the aorta without going to the 

 lungs. But as the lungs become devel- 

 oped, the pulmonary branches increase in 

 proportion to the pulmonary trunk and to 

 the ductus arteriosus. At the termination 

 of fetal life in man, the ductus arteriosus 

 is about as large as either one of the pul- 

 monary branches ; and a considerable por- 

 tion of the blood, therefore, coming from 

 the right ventricle, still passes onward to 

 the aorta without being distributed to the 

 lungs. 



But at the period of birth, the lungs 

 enter upon the performance of the func- 

 tion of respiration, and immediately re- 

 quire a greater supply of blood. The 

 right and left pulmonary branches then enlarge, so as to become the 

 two principal divisions of the pulmonary trunk. (Fig. 313.) The ductus 

 arteriosus at the same time contracts to such an extent that its cavity 

 is obliterated ; and it is finally converted into an impervious cord, 

 which remains until adult life, running from the point of bifurcation of 

 the pulmonary artery to the under side of the arch of the aorta. The 

 obliteration of the arterial duct is complete, at latest, by the tenth 

 week after birth. (Guy.) 



The two auricles are separated from the two ventricles by transverse 

 septa which grow from the internal surface of the cardiac walls ; but 

 these septa remaining incomplete, the auriculo-ventricular orifices con- 

 tinue pervious, and allow the passage of the blood from the auricles to 

 the ventricles. 



The interventricular septum, or that which separates the two ven- 

 tricles from each other, is completed at an early date ; but the inter- 

 auricular septum, or that situated between the two auricles, remains 

 incomplete for a long time, being perforated by an oval-shaped opening, 

 the foramen ovate, allowing, at this situation, a free passage from the 

 right to the left side of the heart. The existence of the foramen ovale 

 and of the ductus arteriosus gives rise to a peculiar crossing of the 

 streams of blood in passing through the heart, which is characteristic 

 of foetal life, and which may be described as follows : 



The two venae cavse in the foetus do not open into the right auricle on 

 the same plane or in the same direction ; for while the superior vena 

 cava is situated anteriorly, and is directed downward and forward, the 



