6SO DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



same time becomes contracted and shrivelled to such an extent 

 that its cavity is obliterated ; and it is finally converted into an im- 

 pervious, rounded 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 hori- 

 zontal septa which grow from the internal surface of the cardiac 

 walls ; but these septa remaining incomplete, the auriculo-ventricu- 

 lar orifices continue pervious, and allow the free passage of the 

 blood from the auricles to the ventricles. 



The interventricular septum, or that which separates the two 

 ventricles from each other, is completed at a very early date ; but 

 the interauricular septum, or that which is situated between the 

 two auricles, remains incomplete for a long time, being perforated 

 by an oval-shaped opening, the foramen ovale, allowing, at this 

 situation, a free passage from the right to the left side of the heart. 

 The existence of the foramen ovale arid 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 : 



It will be found upon examination that the two venae cava?, 

 superior and inferior, do not open into the auricular sac 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 inferior is situated quite posteriorly, and passes into the auricle 

 in a direction from right to left, and transversely to the axis of 

 the heart. A nearly vertical curtain or valve at the same time 

 hangs downward behind the orifice of the superior vena cava and 

 in front of the orifice of the inferior. This curtain is formed by 

 the lower edge of the septum of the auricles, which, as we have 

 before stated, is incomplete at this age, and which terminates 

 inferiorly and toward the right in a crescentic border, leaving at 

 that part an oval opening, the foramen ovale. The stream of blood, 

 coming from the superior vena cava, falls accordingly in front of 

 this curtain, and passes directly downward, through the auriculo- 

 ventricular orifice, into the right ventricle. But the inferior vena 

 cava, being situated farther back and directed transversely, opens, 

 properly speaking, not into the right auricle, but into the left; for 

 its stream of blood, falling behind the curtain above mentioned, 

 passes across, through the foramen ovale, directly into the cavity of 



