DEVELOPMENT OF THE LYMPHATICS OF THE LUNGS IN THE EMBRYO PIG. 65 



of the right lung, while others anastomose with the plexus from the left side, which 

 extends up over the trachea. The development of the lymphatics within the lung 

 depends upon the division of the vessels into two groups — those associated with the 

 veins and connective-tissue septa, and those associated with the arteries and the 

 bronchi. 



The former grow very rapidly, and following each of the branches of the 

 l)ulmonary vein, pass to the pleura. There are at first only two or three lymphatics 

 with each vein. In the early stages the terminal veins lie about midway between 

 the adjacent bronchi, and in this plane a sheet of lymphatics develojjs from the 

 vessels surrounding the \'eins and passes to the jileura, where they mark out the 

 boundaries of the distribution of each bronchus. These vessels anastomose with 

 those that grow direct to the pleura from the plexus on the trachea. 



The bronchial vessels develoji mor(> slowly and at first are to be found only 

 around the larger bronchi. As these structures increase in size and number, the 

 lymphatics surrounding the main bronchi send vessels to the smaller ones and 

 these form a plexus around each of the bronchi, so that the bronchial tree is sur- 

 rounded by a continual series of branching tubes made uj) of lymi^hatic vessels. 

 From every point of division of the bronchi, lymphatic vessels pass to the lymjjhatics 

 of the veins; those around the terminal bronchus leave it near its ending in the atria, 

 and pass to join the lymphatics of the veins or septa, or, more rarely, those of the 

 pleura. 



Lymphatics also arise from the retroperitoneal sac and grow u]i posterior to 

 the diaphragm to enter the lower \)o\e of the lower lobe of the lung. These vessels 

 form a plexus on the median surface of the lower lobe, and send branches both to 

 the pleura of the other surfaces and into the lung along the veins. Plexuses develoi) 

 here as with those that come from above and the two groups soon anastomose. 



The further development consists in the multiplication of the plexuses on the 

 bronchi and blood-vessels, following their continued differentiation. As the lung 

 increases in size, the larger veins become approximated to the bronchi and only the 

 terminal ones are separated from them; these lie in the perijihery of the lobule. 

 Connective tissue is formed along the sheets of lymphatic vessels, and these become 

 the septa of the lung, containing a definite set of vessels which develop fi-om the 

 early vessels following the veins. The lymphatics accompanying the veins remain 

 connected with those of the bronchi and septa. 



The common plexus surrf)unding the artery and bronchus is separated into two 

 individual plexuses, incitlent to the increase in size of the arteiy; however, these 

 continue to have anastomosing branches. 



The vessels of the pleura mark out the early connective-tissue septa, but later 

 there develoi:)s a fine-meshed plexus between these larger vessels, which is not 

 connected with the vessels of the lung-tissue. The valves begin to form at about 

 6 cm. and, in general, point away from the pleura. None, however, have been 

 found in the smaller vessels which accompanj' the terminal bronchi. 



In the adult there are lymphatic vessels accompanying the bronchi, the arteries 

 and the veins; these anastomose freely. There are also vessels in the connective- 

 tissue septa which drain chiefly into those around the veins, and, to some extent. 



