II. On the origin of the pulmonary arteries in mammals

In 1902 I publishcd a p a p a on this subject,' a rcsiiini! of which is here given.The pulrnonary arteries in man, rabbit, cat,, :uid dog, appear as symmetrical vessels, one rising from each fifth, or piilmonary arch.With the growth of the truncus pulmonplis, and its torsion about the bulbus aortq the two pulmonary arches are wound, as it were, around the bulbus, and their malls thus brought into contact are absorbed, so that the truncus pulmonalis grows longer at their expense, the point of bifurcation moving continually farther from the heart.The left arch, being the outside one in this rolling --p proces3, receives the most pull, beconies the strnighkr and therefore tho larger vessel, and is shortened more rapiay.As a rcsult, the point of bifurcation of tlic truiicus pulmonalis reaches the left piiliiionary artery while the right piillnonary artery is still s e a arising froin the right arch some distance dorsal to this point.(See diagram, page 338).The portion of the right pulmonary arch between the origin of thc pulmonary artery and the dorsal aorta becomes obliterated, the anterior portion of the arch remains continiioiis with the artery, and we then have the condition described by Rathke,-the two pulinonnry arteries apparently arising together from the left piilmonary arch.It should be noted, however, that the right pulmonary artery of thc fetus includes, beside the homolognc of thc left pulmonary artery, the proximal portion of the right pulmonary arch.In the pig, although the pulmonary arteries first appear, as iisual, as symmetrical offshoots, one from each pulmonary arch; and although the fetal condition is practically the same, the intermctliatc steps