62 DEVELOPMENT OF THE LYMPHATICS OF THE LUXCJS IX THE EMBRYO PIO. 



In the meantime, the vessels of the pleura, which at from 5 to 6 cm. we have seen 

 beginning to form the true pleural jjIoxus, continue to proliferate, and tlius form a 

 fine-moshod plexus in the pleura between the blocking-off of the lobules. 



The completion of the i)riniary plexus is shown in figure 3, plate 1. This is 

 the surface of the lung in a pig embryo of 6 cm. with the pleural vessels injected. 

 Each of these uninjected areas represents a primary lobule, and the surrounding 

 lyni])hatics mark out tiie connect ive-tis.sue ])lexuses. Figure 1, jjlate 5, illustrates 

 one of the primary lobules, and the close-meshed plexus is the true pleural supi)ly. 

 It is still seen to be connected with the deep vessels of the septum. 



Here and there one finds vessels passing from the terminal bronchi to the sur- 

 face, in the lobule proper, to join with the fine-meshed plexus of the pleura. Tiiese 

 ]iass around the air-cells, but are never found on their walls, and, uniting with the 

 terminal vessels of the end veins, j^ass to join those in the i)leura. These arc the 

 vessels described by Flint (1906) as seeming to dip down into the lobules from the 

 pleura; these, he said, he could follow only a little way into the lobule. This is 

 easily understood from the information gained from injections, for the vessels 

 around the bronchi can not be .seen in uninjected s])ecimens, and consequently those 

 which remain patent in sections seem to end abruptly in the midst of a lobule, 

 whereas they in reality connect with those following the bronchi and terminal veins. 

 The lymphatics that follow the terminal bronchi leave them just before the atria 

 are reached and cross over to join the lymphatics which follow the veins. The 

 lymphatics which accompany the veins pass to the pleura just where the veins bend 

 to reach the center of the lobule. 



Flint first observed the submucous plexus of the bronchi and trachea in 

 embrj^os 23 cm. long. It was surprising that injections did not reveal this j^lexus 

 very much earlier. I have not l:>een able to demonstrate any lymphatics in the 

 submucosa before the embryo reached a length of 19 cm. This plexus develops, as 

 do all the secondary plexuses, by the outgrowth of vessels from the primary one and 

 their coalescence to form the new grouj). This process has been carefully studied 

 by Ileuer in the formation of the niuco.=!al plexus in the intestine. The submucosal 

 plexus is comi)letc just before birth and consists of numerous fine vessels that lie 

 just beneath the bronchial epithelium. From this plexus numerous vessels pass 

 down between the cartilaginous rings and join the lymphatic trunks which follow 

 the bronchi, as has been described. In those bronchi having no cartilaginous rings 

 there is only the one grou]) of lymi)h-ve.s.sels to be found, and these have alreadj' 

 been described. 



The lymphatics of the adult lung were first described by Olaf Rudbeck in 

 1651-1654 (quoted from ]Miller, 1900). Since that time numerous workers have 

 studied the.se vessels. In 1900 W. S. Miller reviewed the literature very thoroughly, 

 and it will, therefore, be unnecessary to repeat that here. Aliller studied the 

 lymphatics in the lungs of adult cats and dogs by injecting them from one of the 

 pleural vessels. He divided the lymphatics into four groups, as follows: 



A. The lymphatics of the bronchi. ('. The lymphatics of the arteries. 



B. Tho lymphatics of 1ho voins. I). Tho lymphatics of the pleura. 



