THE EMBRYOLOGY OF THE BIRD'.S LUNG 33 



sixth united into a oonunon ti'unk are connected witli the cavity 

 of the right ventricle. 



Ah hough the pulmonary arteries are relatively small, the right 

 and left halves of the sixth arch are large, and, since they join the 

 aorta, the principal function of the sixth arch at this time is in 

 connection with the systemic rather than with the pulmonic 

 circulation. 



The pulmonary arteries remain relatively small up to the time 

 of hatching, but those portions of the sixth arch lying behind 

 them become constricted. This condition is represented on the 

 nineteenth day of development in figure 65. At this time the 

 pulmonary arteries are somewhat larger and the portions of the 

 sixth arch behind them has undergone an obvious constriction. 

 The parts of the sixth arch behind the pulmonary arteries are 

 designated each ductus Botalli or ductus arteriosus. In the 

 sketch the carotids and the pulmonics have been cut trans- 

 versely so as to show in each case the cut ends of two vessels 

 instead of single trunks into which they are imited nearer the 

 heart. 



The subsequent steps in the formation of the adult pulmonary 

 arteries involve the disappearance of the ductus Botalli on each 

 side. When respiration begins, the increase of blood flow^ 

 through the lungs reacts on the growing tissue so that the pul- 

 monary arteries become enlarged, and the ducti Botalli being 

 deprived of blood rapidly diminish in size and soon become re- 

 duced to strands of connective tissue which usually disappear 

 in the adult, although in some species of birds they are persistent 

 but not functional. 



The progress of affairs is represented in figure 66 sketched from 

 a chick newly hatched. The pulmonary arteries have increased 

 in size and the ducti Botalli are much reduced. 



After hatching the ducti Botalli become occluded and reduced 

 to slender cords of connective tissue. Figure ()7 represents the 

 condition as seen from the right side, three days after hatching 

 and figure 68 shows the heart and arteries of a young chick of the 

 same age. In figure 68 the connections of the arterial trunks 



THE AMEniCAX JOUKX VI. OF AXATOMV, VOL. 20, XO. 1 



