x.] THE CIRCULATING SYSTEM. 423 



The veins from certain viscera, namely, the spleen, the 

 mesentery, the intestines, and the stomach, finally unite into 

 a single trunk the portal vein, already mentioned. This 

 enters the liver, and there breaks up and ramifies side by side 

 with the ramifications of the hepatic artery, which latter brings 

 arterial blood to that organ. This double supply of blood 

 (one arterial, the other venous) is antagonized by but a simple 

 set of efferent vessels, the hepatic veins. These, as before 

 said, collect themselves together in the liver, meet and en- 

 large, and finally open obliquely into the vena cava inferior 

 a semilunar fold being visible at the lower border of the 

 orifice of each vein. 



There remains one more venous structure which may be 

 noted, namely, what is called the azygos vein an absurd 

 designation, as there are really two such veins, though that 

 on the right side is much the larger. 



The right azygos communicates with the vena cava soon 

 after its own origin, and ascends on the right side of the 

 vertebral column, receiving venous branches called lumbar 

 veins and intercostals. It originates below in the lumbar 

 veins and terminates above in the vena cava superior. 



The left azygos is similar, except in size and in that it 

 opens above into the left innominate vein. 



The two azygos veins communicate by a transverse branch 

 passing behind the oesophagus, and it is here that the left 

 vena azygos is said (in works on human anatomy only) to 

 terminate the part above this junction, and intermediate 

 between it and the innominate vein, being spoken of in 

 Anthropotomy as the left superior intercostal vein. 



9. In their DEVELOPMENT the veins of man undergo re- 

 markable modifications. 



The first veins to appear are a pair which come from the in- 

 testine and ventral region to the heart, and lay the foundation 

 of the portal and hepatic veins. These primitive veins are 

 called ompJialo-meseraic ; they unite and dilate into a venous 

 chamber (called sinus venosus] before entering the heart. 



Two other venous trunks appear on each side, beneath the 

 primitive skeletal axis, and each sends down a vein (the 

 ductus Cuvieri} which opens also into the sinus venosus. 



The part of each trunk which runs backwards from the 

 head to the ductus Cuvieri is called the anterior cardinal 

 vein. The part of each trunk which runs forwards from the 

 hind part of the body to the ductus Cuvieri is called the 

 posterior cardinal vein. 



