274 CIRCULATORY SYSTEM. 



entering, during foetal life, by its largest branch, into the 

 aorta. 



The Ductus Arteriosus constitutes this branch of the pulmo- 

 nary artery, and is, in fact, the continuation of the trunk of the 

 latter into the aorta, immediately behind the origin of the left 

 subclavian artery. The right and the left pulmonary artery 

 at this period, are but inconsiderable trunks, incapable by any 

 means of carrying off all the blood of the right ventricle; the 

 greater part of it, therefore, is conveyed by the ductus arterio- 

 sus into the descending aorta. As the contraction of the ven- 

 tricles, like that of the auricles, is synchronous, it is evident that 

 the column of blood in the descending aorta, is acted upon by 

 both ventricles at the same moment. 



The ductus arteriosus preserves the principle of a single cir- 

 culation in the fcetus, which was first of all manifested by the 

 two ventricles, constituting but one cavity, and by the aorta 

 and pulmonary artery being but one trunk. At the first act of 

 inspiration the lungs, which were before solid, and the thorax, 

 which was compressed, are greatly augmented in volume by the 

 introduction of air. The dilatation of the thorax, besides intro- 

 ducing air through the trachea, causes an increased flow of blood 

 through the right and left pulmonary arteries, in order to fill the 

 vacuum in the lungs. The pulmonary arteries become in that 

 way permanently dilated, and the circulation is finally drawn 

 off entirely from the ductus arteriosus, though this takes several 

 weeks or months before it is completely accomplished. The duc- 

 tus arteriosus in this time is continually contracting, and is at 

 length converted into a ligamentous chord, like other arteries, 

 whose circulation has been arrested. 



These are the several peculiarities which distinguish the foetal 

 circulation, owing to the privation of respiration ; and it is clear, 

 that the collective result is that of a circulation quite as simple 

 as if the heart consisted of but two cavities ; while, at the same 

 time, it keeps this organ in a state of preparation for carrying 

 on two distinct circulations, one pulmonary and the other aortic, 

 from the moment that respiration begins: so that the whole 

 mass of blood is, in subsequent life, brought successively under 



