698 



REPRODUCTION. 



(Fig. 248). One of these secondary trunks becomes the commencement 

 of the aorta, the other the pulmonary artery ; and the relation of the 

 furrow between them to the interventricular septum is such that the 

 aorta communicates with the left ventricle, and the pulmonary artery 

 with the right. V 



But the pulmonary artery, beside supplying small branches on each 

 side to the lungs, also furnishes a large branch of communication to the 

 arch of the aorta beyond. This branch is so voluminous that it appears 

 to be the main continuation of the pulmonary trunk. It forms, accord- 

 ingly, an open canal or duct between the two principal arteries nearest 

 the heart, and is known by the name of the Ductus arteriosus. 



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

 trunk; and nearly the whole of the blood from the right ventricle 

 passes through it to the aorta, only an insignificant quantity being 

 distributed to the lungs. But as the lungs become developed, the 

 pulmonary branches increase in size, though not sufficiently to receive 

 all the blood of the pulmonary trunk. At the termination of foetal 

 life, in man, the ductus arteriosus is about as large as either of the 

 pulmonary branches ; and a considerable portion of the blood, there- 

 fore, coming from the right ventricle, still passes onward to the aorta 

 without being distributed to the lungs. 



But at birth, when the lungs begin the performance of respiration, 

 they receive a greater supply of blood. The right and left pulmonary 

 branches enlarge, so as to become the principal divisions of the pul- 

 monary trunk (Fig. 249). The ductus arteriosus at the same time 



diminishes in size, and is soon obliterated. 

 It remains, in the adult, as an impervious 

 cord, running from the bifurcation of the 

 pulmonary artery to the under side of the 

 arch of the aorta. The obliteration of 

 its cavity is usually completed by the 

 tenth week after birth. (Guy.) 



The interventricular septum, by which 

 the two ventricles are separated from 

 each other, is formed at an early date ; 

 but the interauricular septum remains for 

 a lono* time incomplete, being perforated 



HEART OF INFANT, showing the disap- , . * 



pearance of the arterial duct after by an oval-shaped opening, the foramen 



birth.!. Aorta. 2. Pulmonary artery, ovale, which allows a free passage from 

 3,3. Pulmonary branches. 4. Ductus ,, . . . . , ., , , ml . , 



arteriosus becoming obliterated. the right auricle to the left. The exist- 



ence of the foramen ovale and ductus 



arteriosus gives rise to a peculiar crossing of the streams of blood 

 in the heart, characteristic of fcetal life, as follows : 



In the foetus the two venae cava3 open into the right auricle on dif- 

 ferent planes and in different directions. While the superior vena cava 

 is situated anteriorly, and is directed downward and forward, the in- 

 ferior is situated posteriorly, and joins the auricle in a direction from 



FIG. 249. 



