DEVELOPMENT OF THE LYMPHATICS OF THE LUNGS IN THE EMBRYO PIG. 61 



Now, it will be well to review briefly the state of the development of the lung 

 lymphatics at the time that the primary system is complete — that is, in 6 cm. 

 embryos. At 6 cm. the lymphatics around the trachea form a close-meshed i^lexus 

 near the bifurcation, extending down into the lung around the bronchi. Above the 

 bifurcation there are only a few connecting vessels on the ventral and dorsal surfaces 

 of the trachea, but the two plexuses on the lateral surface are very close-meshed. 

 From the left plexus the principal supply of both Kmgs is derived, but there are 

 numerous vessels passing down into the right lung from the right i)lexus, and the 

 two are closely bound together, especiallj" near the bifurcation, where they have 

 fused into one plexus. The vessels surrounding the bronchi follow them towards 

 the periphery, giving off branches to the venous tree at every division of the bronchial 

 tree. Each smaller bronchus derives its lymphatic supi^ly from the plexus that 

 accompanies the parent bronchus. These vessels are very difficult to inject. 



Accompanying the primary divisions of the pulmonary vein there is another 

 group of vessels that is closel.y l)ound, by anastomoses, to the lymphatics around the 

 principal bronchi (figure 4, plate 1). Along each of the tributary veins vessels pass 

 to the pleura and spread out in the region that has been described as the septa 

 between the lobules. Each of these dividing sheets anastomose with other sheets 

 and with the pleural vessels. The vessels derived from the retroperitoneal sac are 

 continouus with those derived from the two ducts; there can be determined no line 

 of differentiation either within the lung-tissue or on the pleural surface. The poste- 

 rior pole is connected with the retroperitoneal sac by three or four vessels that pass 

 down in the fold of tissue that precedes the ligamentum pulmonale (figure 3, plate 5). 

 The pleural plexus has begun to form within the gross markings that we have 

 described as corresponding to the connective-tissue septa. These vessels are very 

 superficial and are not connected, at this time, with the deeper vessels. 



The further development is chiefly due to the multi])lication of the lung units 

 and the increase in volume of the interbronchial tissue. As new bronchi are formed, 

 new groups of lymphatics bud off from the plexus that accomjianied the parent 

 bronchus and follow the new-formed structure towards the periphery. These 

 lymphatics leave the bronchus and pass to the venous group when thej'^ reach the 

 region where the air-sacs arc developing. 



As the lung-tissue differentiates further and further, the larger veins become 

 more closely associated with the bronchi and only the terminal vessels are peripheral 

 with reference to the lobule. This brings about the relations that are found in the 

 adult, where the princij)al veins and bronchi are closely associated, while the terminal 

 ones have the same relative positions that have been described for the developing 

 structures. 



The arteries in early stages lie very close to the bronchi and are associated with 

 the plexuses that follow that structure. As these blood-vessels increase in size the 

 bronchial plexus differentiates into two parts, following the arteries and the bronchi. 

 This is accomplished by the growth of vessels around the arteries, and, as the artery 

 increases in size, the two plexuses become entirely distinct, but are still connected 

 ]-)y numerous anastomotic vessels. 



