60 DE\ELOPMENT OF THE lA-MriiATlCS OE 'JlIE LUNGS EN THE ENHJRVO I'lG. 



of tissue that passes down behind the diai)hragm to end in the tissue around the 

 aorta; it corresponds to the li<>;amentuni i)ubnonale in the human. It is through 

 this prolongation of tlie lower lobe that the lynii^hatics from the retroijcritoneal sac 

 grow up to reach the lung. These vessels arise from the cephalad portion of the sac 

 and pass up behind the dorsal wall of the stomach to enter this long posterior or 

 lower jiole of the lung (figure^ 2, plate 4). There are three or four vessels that grow 

 out from the sac and vij) into the lung; these are closely associated with those that 

 pass to the diaphragm and, in adult life, join with them just before reaching the 

 nodes into which they drain. They pass upward and divide, on reaching the lung, 

 into two groups, one of which passes up over the diaphragmatic surface and the other 

 over the outer or lateral surface of the lower lobe. 



The anlage of the ligament um pulmonale is connected not only with the lower 

 pole of the lung, but also with the median border of the lower lobe. Thus the 

 lymphatics grow directl}' up al^out one-third of the way to the hilum in this medial 

 extension of the ligament, and from there sweep out in a fork-like division which 

 ])roduces the two plexuses on the two borders of the lung (figure 3, jjlate 5). I have 

 injected these vessels at 3.4 cm.; but I think that they reach the lung border a 

 little earlier. 



From the two plexuses described above vessels grow into the lung in exactly 

 the reverse order to that followed by those developing from the hilum. They grow 

 in just where they will meet the veins, and along these form the septal j)lexuses, 

 exactly similar to those described above. These rapidly anastomose with the other 

 lymphatics, and, by the time the embryo has reached 4 cm. in length, the entire 

 lung is uniformly supplied. 



It is very pertinent to incjuire why the lymphatics that reach tlie lung from 

 below select these points for the invasion of the deeper tissue of the lung. However, 

 when it is recalled that the lymphatic vessels which lie in the mesenchymal tissue 

 (the pleural anlaga) are verj' large in proportion to the other structures and that 

 the budding vessel would be in direct relation to the outgoing veins, it is easily 

 understood that exactly the same causes must be acting here as those which direct 

 the growth from above. So here, as above, the position of the veins controls the 

 direction of growth. Of course, the plexuses on the two surfaces become more 

 complex as the lung is invaded and follow the same steps as the pleural supply in 

 general. As has been said, there are branches along the pleura, and these anasto- 

 mose with the other pleural vessels, so that the supply becomes general. The 

 drainage in the early stages — that is, before the formation of the valves — is probably 

 divided; the flow of lymph might be to the retroperitoneal sac via the vessels that 

 grow u]) from that structure, or to the thoracic ducts through the tracheal ]ilexuses 

 and the vessels accomj)anying the veins and the bronchi. 



We have seen how the lymphatics grow into the lung-tissue and there form two 

 distinct groups, and how one of these rapidly reaches the pleura and there forms the 

 characteristic plexus-pattern marking off the boundaries of the lobules; also how 

 the vessels grow into the posterior j^oles of the lower lobes and anastomose with 

 the system from above, which follows the veins in the connective-tissue septa. 



