THE LYMPHATIC SYSTEM 279 



the umbilical vein within a week. The cord-Hke vein is persistent as the liga- 

 mentum teres of the hver ; the arteries become the obliterated hypogastrics. 



The ductus venosus atrophies because after birth only the blood from the por- 

 tal vein enters the liver, and this is all drained into the liver sinusoids, forming the 

 portal circulation. The ductus venosus is persistent as the fibrous ligamentum 

 venosum, embedded in the wall of the hver. 



The ductus arteriosus ceases to function after birth, as all the blood from the 

 pulmonary arterial trunk is conveyed to the expanded lungs. The ductus be- 

 comes impervious from ten to twenty days after birth and persists as a sohd, 

 fibrous cord, the ligamentum arteriosum. 



After birth, the large amount of blood now returned to the left atrium from 

 the functional lungs equaHzes the pressure in the two atria. As a result, both dur- 

 ing diastole and systole, the septum primum, or valve of the foramen ovale, is 

 pressed against the septum secundum, closing the foramen ovale. Eventually the 

 two septa fuse, though they may be incompletely united during the first year 

 after birth, or even longer (p. 255). 



■-' The Lymphatic System 



The lymphatic system originates in a plexus of lymphatic capillaries distrib- 

 uted along the primitive main venous trunks. By the dilation and coalescence 

 of this network at definite regions five lymph sacs appear (Fig. 287). Paired 

 jugular sacs arise in 10 to 11 mm. embryos lateral to the internal jugular veins. 

 In embryos of 23 mm. the unpaired retroperitoneal sac develops at the root of the 

 mesentery adjacent to the suprarenal bodies, and the cisterna chyli also appears. 

 Paired posterior sacs arise in relation to the sciatic veins in embryos 24 mm. long. 

 These sacs at first contain blood which they soon discharge into neighboring veins, 

 thereupon losing their venous connections. With relation to the lymph sacs as 

 centers, the thoracic duct (at 30 mm.) and the peripheral lymphatics develop. 

 The jugular sacs alone acquire with the internal jugular veins secondary con- 

 nections which are later utilized by the thoracic and right lymphatic ducts. The 

 various sacs themselves are eventually transformed into chains of l3Tnph nodes. 



Two discordant views exist as to the origin of the lymphatics. According to 

 Sabin (Amer. Jour. Anat., vols. 1, 1901; 9, 1909) and Lewis (Amer. Jour, Anat., vol. 

 5, 1906), sprouts arising from the endothelium of veins form the single and paired 

 sacs already described. From these five sacs the thoracic duct and peripheral 

 lymphatics develop as endothelial outgrowths. Thus lymphatic vessels grow to 

 the head, neck, and arm from the jugular sacs; to the hip, back, and leg from the 



