52 DEVKT-OPMENT OF TTTK T.YMPIIATICS OF THE I>U\GS IX THE EMRRYO PIG. 



then apjiioacliing the end blanches the\' leave them and run in a plexiform manner midway between 

 the broncliial tubes until they reach the pleura. This gives the lung now an indefinitely lobulated 

 appearance in which the perijihcry of the simple lobule is indicated by the lymphatic vessels and the 

 center by the l)ronchi. The lymjihatics are lined with flattened eudotlieliuin; their walls are formed 

 by the young connective-tissue fibrils, and here and there valves are beautifully shown which, in 

 general, point away from the pleura. 



Stage 13 cm.: The lymphatics, forming a plexus around the l)ronchial veins and arteries at the 

 root of the lung, accomijany them towards the perijiherj', giving off branches to the interlobular 

 spaces en route. * * * On reaching the peri])hery of the lung they leave these structures and 

 pass out, as in the preceding stages, to the pleura. Thej' have a plexiform arrangement and may be 

 traced at times into the substance of the lobules. This course may be observed in the deeper lobules 

 of the lung as well as in those on the surface under the pleura. 



Stage 19 cm.: In general the relations of the lymphatic system have not changed. 



Stage 23 cm.: At 23 cm. the first evidence of the submucous lymphatic system is seen in the 

 stem bronchi. It may, however, be found earlier, but the vessels are difficult to follow. It would 

 seem thus that we have in the pig's lung, besides the lymphatic plexuses accompanying the bronchi, 

 arteries, and veins, an iiiterloliular system which Miller has lieen unable to find in the human lung. 

 Injections pointing to such a relationship he has iiiteri)reted as artefacts. If Miller's conclusions 

 prove correct, then the Ij'mphatics of the human lung must develop, so far as the interlobular system 

 is concerned, in some other way. 



I quote at length from Flint because he alone, of the many workers on lung 

 lymphatics, has approached the subject from the embryological side. As I have 

 said, Flint was seriously handicajiped by having onlj^ sections from which to draw 

 his conclusions. He was especially struck by the prominence of the vessels lying 

 in the interlobular septa, and attempted to explain their apparent change of course 

 {i. e., from the bronchi to the septa) by the theory that the density of the tissue was 

 greater around the bronchi and vessels and that the lymphatics chose the path of 

 least resistance. He did not call attention to the relation of the veins to this point 

 in the development of the lymphatics, which will be discussed later, but emphasized 

 the fact, so amply shown by injections, that these interlobular vessels grow much 

 more rapidly than the vessels around the bronchi and arteries. 



It will be necessary hereafter to discuss the work of Miller on the adult lym- 

 phatic system, in connection with the later stages; therefore it will suffice to refer 

 here to the statement which Flint discussed in the quotation given above. Miller 

 has called attention to the fact that the terminal vein lies in the periphery of the 

 lobule and that the lymphatics accompanying the vein communicate with those of 

 the pleura. He cites Councilman's (1900) description of the interlobular vessels, but 

 does not claim to have found the same vessels. I think that these different views 

 will be reconcilal:)lc when we have followed the develoi^ment of the lymphatics 

 through the various stages that lead to the adult form. The literature on the 

 lymphatics of the adult mammalian lung is very large, and for a comprehensive 

 review of it the reader is referred to the papers of Miller (1893, 1896, 1900, 1902, 

 1911). It seems needless to discuss it more at length here. 



THE VESSELS ARISING FROM THE LEFT DUCT. 



As has been said, the lymphatics of the lungs arise partly from the two thoracic 

 ducts by sprouts. These vessels grow to the mesenchAniial wall of the trachea and 

 form there a plexus which sends vessels down into the lungs. Other vessels grow 

 directly into the lungs. 



