58 DEVELOPMENT OF THE LYMI'IIATICS OF THE LUNGS IN THE EMBRYO I'lO. 



As the lung increases in size and the veins and bronchi which we have termed 

 secondary give off other branches, these in turn become the terminal ones and 

 assume the relations that have been descrilx-d. The others are, bj' the increasing 

 amount of lung-tissue, forced closer together. Thus it is seen that it is only the 

 terminal veins that occupy the position described; that is, pass along the periphery 

 of the lobule. In the pig there is considerable connective tissue forming definite 

 lobules in the adult lung; and these septa, bounding as they do the area sui)plied 

 by terminal bronchi, divide the lung into a large number of irregular cones or pyra- 

 mids, which have the bronchus and artery in the center and the veins passing along 

 the periphery imtil close to the apex, where they enter veins of the next larger size. 

 For further discussion of this arrangement see Miller's article (1900). 



As we have seen, a few dilated lymphatics are found in the hilum of the lung 

 at 2.9 and 3 cm. These are the first branches from the vessels that are forming the 

 ])lexus on the trachei and bronchi already described. The bronchi, as has been said, 

 are surrounded by lymphatics which follow them into the lung-tissue; and, as 

 secondary bronchi are formed, l>'miihatics from these plexuses branch off to follow 

 them. 



The primary veins lie very close to the corresponding bronchi at this stage, and 

 are accompanied by a few lymphatic trunks which arise from the same general 

 i:)lexus that covers the bifurcation. These vessels anastomose very richly with 

 those of the bronchi, and, close to the point where the trachea divides, they merge 

 together. We have seen that the secondary veins lie midway between the adjacent 

 bronchi, and represent the outer border of the primitive lobule of the developing 

 lung. Along these veins the lym])hatics grow towards tlu^ i)leura; they are derived 

 l)()th from the plexus that follows the ])rimary vein and from the vessels that sur- 

 round the i)rimary ])ronchi. Tlu^ lymphatics from the bronchial supply join those 

 from the vein, and the combined group passes along the vein, spreading out on either 

 side to form a sheet, until the vessels reach the pleura. Flint observed these sheets 

 of lymphatics, but thought that there must be some difference in the density of the 

 tissues to account for their leaving the bronchi to run midway between, lie did not 

 recognize the relation between the veins and the lymphatics. It will be clear, when 

 it is remembered that the smaller branches of one vein spread out fan-like to meet 

 those of the other vein, that the sheets of lymphatics lying between the bronchi 

 are directed by the veins as well as the separate lymph-vessels directly associated 

 with them. 



In this manner the true primitive lobules are formed by the interpolation of a 

 sheet of rapidly growing lymphatics between the bronchial tubes. It is along the 

 distal margin of these plexuses that the pleural marking begins. When these vessels 

 reach the pleura there is a marking-out of the characteristic coarsely-meshed plexus, 

 each interspace corresponding to the sheet beneath (figure 3, plate 1). It must be 

 remembered that these vessels, growing as they do very rapidly, reach the pleura 

 very early, and hence the pleural plexus is developing while the above-mentioned 

 interlobular plexuses are forming. We have so far described only the formation of 

 the large parallel plexuses shown in figure 1, plate 4, figure 2, plate 5, and figure 1, 

 plate 3. But the formation of veins in other planes directs the growth of the lym- 



