THE VASCULAR SYSTEM 1 87 



as in the cat, there are two openings in both the right and left 

 external jugular veins. However, the jugulo-subclavian tap 

 may be absent on one or both sides. Jugular lymph sacs 

 connecting the lymphatic and venous systems in the jugulo- 

 subclavian region have been described by Huntington and 

 McClure as usually present, more or less well-defined, in adult 

 cats. This interesting survival of an embryonic structure has 

 also been found in man and other mammals. In amphibians 

 and reptiles, lymph sacs and pulsating lymph hearts are normal 

 adult structures. In addition to the cervical communications 

 other venous taps may occur in the trunk region, as has been 

 shown by Silvester for monkeys and Job for rats. Lumbar 

 ''taps" have also been found in cats. The significance of these 

 venous taps has been worked out by McClure and Silvester, 

 and more recently, in the rat, by Job. 



DEVELOPMENT OF THE LYMPHATIC SYSTEM 



The lymphatic system like the bloodvascular system is 

 developed in the mesenchyme by the enlargement and fusion of 

 intercellular spaces. According to Huntington and McClure 

 (Anat. Record, IX, 571) ''the development of the thoracic 

 ducts and the mesenteric lymphatics in the cat is correlated with 

 the degeneration of certain venous channels, many of which 

 are tributaries of the azygos division of the supracardinal veins. 

 A series of independent lymph spaces arise discontinuously in 

 the mesenchyme external to the intimal lining of these degenerat- 

 ing vessels and, as these lymph spaces gradually become con- 

 crescent to form continuous channels, the latter following a Une 

 of least resistance, utilize the static line vacated by these degen- 

 erating veins. This extraintimal replacement accounts for the 

 sinistral drainage plan finally assumed by the thoracic duct 

 system in the embryo of the cat." Not all of the lymph channels 

 are related to degenerating veins in this way. As a matter of 

 fact, perhaps the usual method of development is by the 

 concrescence of mesenchymal spaces independent of venous 



