64 DEVF,I,OPMEXT OF THE LYMPHATICS OF THE LUXOS IX THE EMBRYO PIG. 



bution to the venous trunks. The fact that so careful an observer as Miller does 



not find these lymphatics in the sepia suggests the possibility that the assumption of 

 mature activities in some way brings about the atrophy of all of the interlobular 

 lymphatics except those that accompany the veins. Again, this plexus may be 

 peculiar to the pig. It seems necessarj' that this ciuestion must remain unsettled 

 until studied b}' some method other than simi)le injection. 



The ([uestion of the drainage of the lung lym])hatics is of exceptional interest 

 and importance; and while we must dei)end, for the final settlement, upon phj'sio- 

 logical methods, there is much evidence available from morphological observations. 

 In the larger vessels on the bronchi, the veins, and the arteries there are valves 

 which i)oint towards the hilum. This is assumed to be ^•ery goocl evidence that the 

 flow is in that direction. No valves have been described in the Ij'mph-vessels 

 which accompany the smaller bronchi, veins, and arteries. Hence it can not be 

 stated whether the lymi)h flow, in the lymph-vessels of the veins, is towards the 

 ])leura of the hilum; and, in like manner, the flow in the bronchial vessels might be 

 either towards the hilum or towards the veins and arteries. With regard to the 

 vessels on the pleura, all of the lymphatics above a certain regional level of the lower 

 lobe drain either towards the mid-line and then course up in the pulmonary ligament 

 to end in the nodes at the hilum, or pass by direct paths to these nodes. Those 

 below this level drain to the nodes lying in the mesentery of the lesser curvature of 

 the stomach, h-'ome of these drain as do those above — towards the median and 

 pass down in the ligamentum ))ulmonale — while others i)ass directly down from the 

 ]K)sterior pole. This group of vessels which pass to the preaortic nodes drains 

 about one-third of the lower lobe of the lung. This varies considerably; in some 

 specimens as much as half of the lung has been found to drain in this direction. 



This peculiar drainage of the lower lobes seems especially important from the 

 bearing that it may have on the pathology of the lungs. It has long been known 

 that the diaphragmatic vessels drain to these nodes, but there is no connection 

 between these vessels and those of the lung ])ro])er. The lymphatics that jiass 

 through the pulmonary ligament apparently drain only the pleura; but, as has been 

 shown, the deep lymphatics anastomose with those of the pleura, and therefore it 

 seems possible for substances to pass from the lung-tissue to the preaortic nodes. 

 What bearing this may have u]ion the pathology of the lungs or of the abdomen 

 remains to be settled. 



SUMMARY. 



The hniphatics of the lungs are derived from three sources — the right and the 

 left thoracic ducts and the retroperitoneal sac. 



In embryos 2.G to 3 cm. long, vessels bud off from the thoracic duct and grow- 

 across to the trachea, forming there a plexus that gradually extends over the ventral 

 surface of the trachea, and especially down over the bifurcation. From this plexus 

 vessels pass into both lungs and into the pleura. 



The right thoracic duct divides, in embiwos about 2.5 cm. long, into two vessels. 

 One passes to the heart, while the other breaks up to form a plexus on the right 

 lateral wall of the trachea. Some vessels from this plexus pass down into the hilum 



