138 On the Origin of the Puhnonary Arteries in Mammals 



are seen. As the arteries increase in length their proximal ends, where 

 they arise from the aortic arches, seem to approach each other actually, 

 as can be seen by comparing Figs. 1, 2 and 3. The mechanism of this 

 change is probably as follows: the truncus pulmonalis is at first short, 

 soon dividing into its two branches, the right and left fifth aortic arches; 

 as it becomes twisted around the aorta, following the turn of the heart, 

 the truncus pulmonalis pulls on the two fifth arches, which are thus 

 crowded together, forming a double tube, and at the same time the two 

 pulmonary arteries, arising from the mesial aspect of the two arches, are 



Fig. 1. 



Fig. 



Fig. 1. Kabbit of 5.0 mm. Frontal series No. 14S, sections 250-261. 

 X 80 diameters. A, B, left and right pulmonary arches, opening directly 

 into heart, H. C, D, pulmonary arteries. F, G, fourth aortic af-ches, 

 opening into heart. 



.Fig. 2. Eabbit of 8.0 mm. Frontal series No. 154, sections 291-311. X 80 

 diameters. E, junction of A and B. H, valve of heart. 



brought nearer together. By fusion of the two parallel arches the 

 truncus pulmonalis is increased in length, and its two branches short- 

 ened; this fusion may extend until the origins of the pulmonary arteries 

 are very near the bifurcation, or until the left artery springs actually 

 from the bifurcation. 



The diameter of the pulmonary arteries remains small in comparison 

 to their increasing length, as one might expect from the slight necessity 



