48 DEVELOPMENT OF THE SVSTEMIC LYMPHATIC VESSELS 



metrical stage. This occurrence leads to the well-known relative 

 location of the main axial veins and lymphatics in the normal 

 adult, in which the lymphatic vessels are chiefly situated on the 

 left side and form, so to speak, a mirror-picture of the right sided 

 axial venous channels. 



Fig. 27 shows the reconstruction of the anterior venous and 

 lymphatic complex in a cat embryo of 18 mm. (series 88) in the 

 ventral view, and fig. 28 of the same preparation in the lateral 

 asi)ect from the left side. The brachiocephahc cross anastomosis 

 is already well under way, resulting in a marked diminution of 

 the left anterior caval vessel and a corresponding increase in the 

 permanent right anterior cava or right duct of Cuvier. Con- 

 versely, the Ijanphatic vessel accompanying the diminishing left 

 precaval vein is of large size, while that applied to the massive 

 riglil precaval is comparativelj^ small. 



This i)rincii)le of lymphatico-venous replacement, indicated 

 clearly in the later embryonic stages, is strikingly illustrated in 

 the adult. Thus, for example, the adult cat presents normally 

 the arrangement of the great veins of the head and neck which 

 is so freciuentlj' encountered in mammals below Primates, in 

 which the large embryonic internal jugular vein is much reduced 

 or even entirely obliterated, while secondarily the external jugu- 

 lai- vein has assiuned the function of the main vessel. Under 

 these normal circumstances the lymphatic trunk accompanying 

 the minute internal jugular vein or, in case of its entire default, 

 occuiA'ing its }X)sition, is well developed and the largest element 

 of the entire cervical lymjihatic complex, while the external jugu- 

 hii- vein is, on the other hand, accompanied usually by two very 

 slender lym])halic vessels. 



in instances, however, in which the embryonic proportion 

 hciuccii tlio two jugular veins is retained in the adult, so that the 

 internal jugular a])pears as a large and functionally important 

 vessel, w hile the external is correspondingly diminished, the inter- 

 nal jugular lymi)hatic trunk is reduced, while the double lymphatic 

 vessel along the external jugular is enlarged, and evidently acts 

 in ('(jnipensation in the cervical lymphatic return. 



