902 HUMAN ANATOMY. 



consequently will not prevent a flow of blood from the vena cava outward to che 

 ascending lumbar veins in cases of occlusion of the upper part of the ve^ia cava. 



3. The Hepatic Veins. — The hepatic veins (vv. hepaticae) (Fig. 765) return 

 the blood which has been carried to the liver both b)' the hepatic artery and 

 by the portal \ein. They are two or three in number, and are formed by the 

 union of the intralobular veins of the liver (page 920 J). They emerge from the 

 substance of the liver at the upper part of the groove in which the vena cava 

 lies, and, passing obliquely upward, enter that vessel at an angle shortly before it 

 passes through the diai:>hragm. 



One of the hepatic veins drains the substance of the right lobe of the liver, 

 the other, when there are but two, the remaining lobes. Quite frequently this 

 second or left vein is replaced by two vessels, one of which drains the left 

 lobe alone, while the other drains the Spigelian and quadrate lobes. Usually, 

 in addition to these principal \'eins, a varying number of small hepatic veins 

 occur, which make their exit from the liver-substance on the walls of the groove 

 for the vena cava and open directly into that vessel without joining the principal 

 hepatic veins. 



The hepatic veins possess no valves in the adult, and are characterized by the 

 thickness of their walls, which are provided with both circular and longitudinal 

 muscles. 



Variations. — Occasionally the right vein, more rarely the left, perforates tlie diaphragm 

 and opens either into the thoracic portion of the inferior vena cava or else directly into tfie right 

 auricle. The two (or three) veins sometimes unite to a single trunk before joining the vena 

 cava, and this trunk has been observed to penetrate the diaphragm and open directly into the 

 right auricle without communicating with the vena cava. 



4. The Renal Veins. — The renal veins (vv. renales) (Fig. 766) are two in 

 number, one returning the blood from each kidney. Each vein is formed at the 

 hilum, or some little distance from it, by the union of from three to fi\'e branches 

 which come from the kidney substance, and is directed medially and slightly upward, 

 lying in front of the corresponding artery. On account of the position of the vena 

 cava to the right of the median line, the left vein is somewhat longer than the right, 

 and passes in front of the abdominal aorta, just below the origin of the superior 

 mesenteric artery, to reach its point of entrance into the vena cava, this point being 

 usually a little higher than that of the right vein. 



Tributaries. — hi addition to the vessels by whose union it is formed, each renal vein receives 

 (a) an inferior suprarenal vein from the lower part of the suprarenal capsule, accompanying the 

 corresponding artery ; [b) adipose veins, which pass transversely across both surfaces of the 

 kidney, taking their origin in its adipose capsule; (r) a ureteric vein, frequently more or less 

 plexiform in structure, which returns the blood from the upper part of the ureter, anastomosing 

 below with the ureteric tributaries of the spermatic \-ein. In addition, the left renal vein receives 

 the left spermatic (ovarian) and the left middle suprarenal veins, both of which will be 

 considered with their fellows of the opposite side. 



The adipose veins ramifying in the kidney fat penetrate the renal fascia and so come 

 into connection with the tributaries of the lumbar veins, and they also send branches to the 

 spermatic or ovarian veins. A more important communication is, however, made through 

 a vein which arises from the lower surface of each renal and empties on the right side 

 into the first lumbar vein, while on the left side it bifurcates, sending one branch downward 

 to the first lumbar and the other upward to open into the hemiazygos. Since valves occur 

 but rarely in the renal vein, and its tributaries are likewise either without valves or with 

 insufficient ones, the circulation of the kidney may be maintained by means of these 

 communications of the renal veins, even in cases of obliteration of the vena cava inferior in 

 its upper portion. 



Variations. — The renal veins are occasionallv replaced by from two to seven vessels which 

 open indejiendently into the vena cava, — a conditiiMi which probably depends upon the failure of 

 the vessels from the different portions of the kidneys to unite to a common stem. Accessory 

 veins, which communicate with the vena cava below the level of the renals or even with the com- 

 mon iliac, sometimes occur, but more rarely than the similar arteries. The left renal vein has 

 been observed in several cases to pass almost vertically downward parallel to the vertebral 

 column, opening into the vena cava at the level of the fourth lumbar vertebra 



