DEVELOPMENT OF THE LYMPHATICS OF THE LUNGS L\ THE EMBRYO PIG. 55 



cardiac division, after reaching the base of the heart, along the posterior wall of 

 the vena cava, i)asses around the bulbous arteriosus to reach the anterior surface 

 of the heart, where it divides to form the primitive pericardial plexus. By intro- 

 ducing a canula dorsal to the vena cava and injecting towards the heart, I was able 

 to fill this plexus in a pig 3 cm. long. At this stage it extends about one-fourth of 

 the distance from the base to the apex of the heart. Figure 13 in Volume V of 

 the Johns Hopkins Hospital Reports, Monograph Series (Sabin on "The Origin and 

 Development of the Lymphatic System"), shows the right duct near the heart in an 

 embryo pig 2.5 cm. long. In that paper attention was called to the fact that the 

 duct grows towards the heart and that it probably represents the cardiac supplj'. 



The second of the two terminal branches of the right duct passes down parallel 

 to the dorsal wall of the trachea in about the same general position as that occupied 

 by the duct above the point of division. Thus it might seem proper to consider the 

 lung division as the more fundamental of the two, as it appears to be the continua- 

 tion of the undivided duct. However, the heart branch is probablj' the more fun- 

 damental and the earlier of the two, since it is a general principle in the growth 

 of lymphatic trunks for the principal vessels to follow the larger blood vascular 

 channels. Hence we consider the left duct as primarily aortic and the right as 

 primarily cardiac in distribution. 



This vessel enters the hilum of the lung and breaks up into a few branches that 

 are grouped around the bronchi and blood-vessels as on the left. The nature of the 

 grouping and the further development are similar on the two sides, and hence both 

 will be considered together. There is, however, an interesting difference between 

 the two upper lobes, which is dependent upon the relation of the aortic arch to the 

 hilum on the left. On the right the lung is distinctly higher {i. e., nearer the neck) 

 than on the left, because on the latter side the aortic arch lies in the groove made at 

 the juncture of the upper lobe with the trachea. Thus the vein to the upper lobe 

 on the left passes close to the bronchus under the aortic arch, while on the right it 

 is well above the bronchus. This allows more freedom in the lymphatic growth on 

 the right, so that the vessels to the upper lobe come down directlj' into it instead of 

 growing back from a single group, as they do on the left. It must be understood 

 that the stage referred to is between 2.5 and 3 cm., when the heart is still higher 

 than the bifurcation.. Later the heart passes still farther down into the thoracic 

 cavity, and these differences disappear as the cardiac and aortic relations to the lung 

 begin to assume their adult form. There is, however, one very important effect 

 of this asymmetry; the lymjihatics of the right duct pass directly into the lung, 

 while those of the left must course up over the arch of the aorta and the bifurcation 

 of the trachea to reach the lung-tissue. This has been mentioned briefly before. 

 It is clear that the principal supply of the bronchi, and therefore, ultimately, of the 

 lungs, comes from the left duct. This is in large measure the result of the asym- 

 metric relations of the heart and aorta. 



The development of the first vessels to the trachea and lungs on the right side 

 will next be described in detail. From the heart limb of the right duct a few vessels 

 arise and grow down over the vein to the upper lobe on the right side; after crossing 

 the vein they enter the lung near the hilum and di^•ido into several branches, some 



