LYMPHATIC SYSTEM 333 



postcaval : as the allantois increases in size, it brings back the 

 oxygenated blood from this organ (i.e., from the placenta in the 

 higher Mammalia). The right umbilical vein, however, early be- 

 comes obliterated, and the left comes into connection with the 

 capillaries of the liver, its main stem in this region disappearing 

 (Fig. 270, b). Thus the blood from the allantois has to pass through 

 the capillaries of the liver before reaching the heart. In the course 

 of development, however, a direct communication is formed be- 

 tween the left umbilical vein and the remains of the fused vitelline 

 veins, and this trunk is known as the ductus venosus (Fig. 270, c). 

 On the cessation of the allantoic (or placental) circulation, the 

 ductus venosus becomes degenerated into a fibrous cord, so that 

 all the portal blood has to pass through the capillaries of the liver. 

 The intra-abdominal portion of the umbilical vein persists 

 throughout life as the epigastric vein in Reptiles and in Echidna, 

 but disappears in Birds and in other Mammals. 



The mode of development of the veins of the extremities is essentially 

 similar in all the Amniota, and at first resembles that occurring in 

 Amphibia, though later on considerable differences are seen in these two 

 groups, more especially as regards the veins of the digits. 



Retia Mirabilia. 



By this term is understood the sudden breaking-up of an arte- 

 rial or venous vessel into a cluster of fine branches, which, by 

 anastomosing with one another, give rise to a capillary network ; 

 the elements of this network may again unite to form a single 

 vessel. The former condition may be described as a imipolar, the 

 latter as a bipolar rete mirabile. If it is made up of arteries or of 

 veins only, it is called a rete mirabile simplex ; if of a combination 

 of both kinds of vessels, it is known as a rete mirabile duplex. 



The retia mirabilia serve to retard the flow of blood, and thus 

 cause a change in the conditions of diffusion. They are extremely 

 numerous throughout the Vertebrate series, and are found in the 

 most varied regions of the body, as, for instance, in the kidneys 

 (glomeruli, p. 345)- — where their above-mentioned function is most 

 clearly seen ; on the ophthalmic branches of the internal carotid ; 

 on the vessels of the air-bladder in Fishes (p. 280) ; along the 

 intercostal arteries of Cetacea ; on the portal vein ; and along the 

 caudal portion of the vertebral column in Lizards. 



Lymphatic System. 



In Fishes, Amphibians, and Reptiles, but more particularly in 

 the first-named Class, lymph vessels (p. 299) are often not 

 plainly differentiated, and occrir mainly along the great blood- 



