DEVELOPMENT OF THE LYMPHATICS OF THE LUNGS IX THE EMBRYO PIG. 57 



It is of importance to note here that the heart is migrating downwards (i. e., 

 caudalwards) during this period, and, by the time the embryo has reached 4.5 cm. 

 in length it has come to lie almost directly over the hilum of the lung. Hence the 

 vessels that formerly ran in a long course from their point of origin in the heart 

 limb of the right thoracic duct to reach the upper lobe and the hilum of the lung 

 have become a part of the common tracheal plexus, and the formerly distinct duct 

 to the heart has also been absorbed by the plexus over the bifurcation. 



The cardiac vessels then (at 4.5 cm.) drain directly into the plexus over the 

 hilum of the lung (figures 1 and 3, plate 2) . This relation remains in the adult in 

 the drainage of the cardiac vessels into the mediastinal nodes and the union of the 

 efferent trunks of these nodes with those from the hilum of the lungs. 



Here must be mentioned, though not bearing particularly on the lymphatics 

 of the lungs, the connection between the right and the left ducts. In specimens of 

 about 3.5 to 4 cm. in length, I have regularly found a vessel arising from the dorsal 

 part of the right tracheal plexus and joining the thoracic duct behind the aorta. 

 As has been said, it seems best to consider the vessel to the heart as the continuation 

 of the right thoracic duct; hence this vessel must be considered, as was the one to 

 the lung, as a part of the collateral supply. 



The lung, as has been stated, also derives lymphatics from another source — ■ 

 the cephalad portion of the retroperitoneal sac. These vessels are growing into 

 the lung during the period when those already described are differentiating, but it 

 seems best to postpone the discussion of this portion of the pulmonic supply until 

 we have studied the early changes that take place in the lung itself, following the 

 invasion by the vessels already described. The desirability of this is evident when 

 it is remembered that the vessels from below must follow a similar course in the 

 lung, with the exception that this course is reversed, due to the fact that these ve.ssels 

 invade the lung through the pleura instead of the hilum, and must reach the other 

 supply through the interlobular septa, to be described later. 



At 3 cm. there are two primary bronchi and two veins on either side, one of 

 each to each upper lobe and one to each lower lobe. From these the secondary 

 branches are beginning to form. From 3 cm. to 5 cm., these secondary branches are 

 developing rapidly and are very large in comparison to the size of the lung. The 

 arteries are very much smaller, and the veins are somewhat larger than the arteries, 

 but much smaller than the bronchi. It is of great importance to note the relations of 

 these structures to each other during this period. Flint has studied their develop- 

 ment very thoroughly, but he does not call attention to the fact, so important with 

 reference to the lymi)hatics, that the developing vein is separated as widely as 

 possible from the bronchus with which it is morphologically associated. The artery, 

 on the other hand, follows the bronchus very closely and is distributed with it to the 

 center of the developing lobule. The two primary branches of the pulmonary vein 

 lie close to the corresiwnding bronchi. This is, indeed, as far separate as is possible, 

 since there is almost no lung-ti.ssue at this period, while the secondary vessels which 

 may be considered the terminal branches lie about equidistant from the two adjacent 

 bronchi. The arteries follow the bronchi more closely. This fact is of the greatest 

 importance in the development of the lymphatics and also in the relation of the veins 

 to the periphery of the lobule in the adult, as has been shown by ;\Iiller (1900). 



