DEVELOPMENT OF THE HEART. 



679 



Fig. 269. 



F<KTAL HK ART still farther 

 developed. 1 Aorta. 2. Pul- 

 monary artery. 3, 3. Pul- 

 monary branches 4. Ductus 

 arteriosus. 



Very soon a superficial line of demarcation, or furrow, shows 

 itself upon the external surface of the heart, corresponding in situa- 

 tion with the internal septum; while at the root of the arterial 

 trunk this furrow becomes much deeper, and finally the two lateral 

 portions of the vessel are separated from each other altogether, in 

 tEe immediate neighborhood of the heart, 

 joining again, however, a short distance be- 

 yond the origin of the pulmonary branches. 

 (Fig. 269.) It then becomes evident that 

 the left lateral division of the arterial trunk 

 is the commencement of the aorta ( i ); while 

 its right lateral division is the trunk of the 

 pulmonary artery (2), giving off the right 

 and left pulmonary branches (3,3), at a short 

 distance from its origin. That portion of 

 the 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 

 ventricle, passes directly onward through the arterial duct, and 

 enters the aorta without going to the lungs. But as the lungs 

 gradually become developed, they require a larger quantity of 

 blood for their nutrition, and the pulmonary branches increase in 

 proportion to the pulmonary trunk and the ductus arteriosus. At 

 the termination of foetal life, in the 

 human subject, the ductus arteriosus 

 is about as large as either one of the 

 pulmonary branches ; and a very con- 

 siderable portion of the blood, there- 

 fore, coming from the right ventricle 

 still passes onward to the aorta with- 

 out being distributed to the lungs. 



But at the period of birth, the lungs 

 enter upon the active performance of 

 the function of respiration, and imme- 

 diately require a much larger supply 

 of blood. The right and left pul- 

 monary branches then enlarge, so as 

 to become the two principal divisions 

 of the pulmonary trunk. (Fig. 270.) The ductus arteriosus at the 



Fig. 270. 



HEART OF IXFAXT, showing dis- 

 appearance of arterial duct after birth. 

 1. Aorta. 2. Pulmonary artery. 3, 

 3. Pulmonary branches. 4. Ductus 

 arteriosus becoming obliterated. 



