ON THE ORIGIN OF THE PULMONAEY AETERIES IN 



MAMMALS. 



BY 



JOHN LEWIS BREMER, M. D. 

 From the Embryological Laboratory of Harvard Medical School. 



With 9 Text Figukes. 



The material used in prepai'ing this paper is from the collection of 

 the Laboratory of Embryology at the Harvard Medical School; the origi- 

 nal numbers of the series and sections have been preserved. The draw- 

 ings are from reconstructions, and represent, as it were, casts of the 

 lumina of the arteries without reference to the thickness of their walls. 

 They are all of the same magnification (X 80 diameters); the arteries 

 are seen from behind, and the pulmonary arches can be followed until 

 they unite to form the truncus pulmonalis, or until, as in Fig. 1, they 

 enter the heart itself. 



In 1857, H. Rathke published his monograph, "Die Aortenwiirzeln 

 und die von ilinen ausgehenden Arterieii der Saurier," in which appear 

 the diagrams of the aortic arches now made more familiar by their repro- 

 duction by Kolliker, Hertwig, Quain, and many others, Avith or without 

 slight modifications. In these diagrams the right and left pulmonary 

 arteries are represented as arising, in lizards and birds, from their re- 

 spective fifth, or pulmonary, arches, while in snakes and in mammals 

 one fifth arch alone gives rise to both pulmonary arteries, the other arch 

 becoming obliterated; in snakes the right pulmonary arch remains, in 

 mammals the left. Since this monograph there has been, so far as I 

 know, no special investigation into the origin of the pulmonary arteries. 



The earliest buds of the pulmonary arteries, in the rabbit, appear in 

 embryos of about 4.0 mm., one bud from each of the puhuonary arches, 

 on the mesial aspect of each. The growth of these buds is at first back- 

 ward, then downward and inward, giving a small twist, Figs. 1, 2, 3, x, 

 near the proximal end of the pulmonary artery, which seems peculiar to 

 the rabbit. From this twist, the course is straight downward, on each 

 side of the trachea and slightly anterior to it, to the lungs, where the 

 usual branches are given off. During this downward course no branches 



