54 COMPARATIVE ANATOMY 



wide vessel which runs into an anterior angle of the sinus 

 venosus. The systems of the right and left venae cavae 

 anteriores are similar, and the above description serves for 

 either side of the body. 



There are, then, two anterior ven^ cavas opening into the 

 two anterior angles of the triangular sinus venosus. But there 

 is only one posterior vena cava opening into the posterior 

 angle of the sinus venosus and bringing back blood from the 

 abdominal viscera, the trunk, and hind limbs. The system of 

 the posterior vena cava is complicated. It arises between the 

 kidneys, where it is formed by the union of four or five renal 

 veins from each side, and by the ovarian or testicular veins in 

 the female or male. It passes straight forwards in the median 

 line, is partly enveloped by the substance of the liver, and, just 

 before it enters the sinus venosus, it receives the right and 

 left hepatic veins from the liver. The blood from the hind 

 limbs, and from the other abdominal viscera, reaches the vena 

 cava only after an indirect course through either the liver or 

 kidneys. 



The blood from the hind limb is returned to the pelvic 

 region by two veins — the femoral, which twists round from back 

 to front of the thigh to reach the ischial region of the pelvis, 

 and the sciatic, which comes up from the back of the thigh to 

 pass under the iliac bone of its side. The sciatic vein is 

 almost directly continuous with a vein known as the renal 

 portal vein, which passes forward to the outer border of the. 

 kidney, and there breaks up into small vessels which open into 

 the capillaries of the kidney. The femoral vein divides at the 

 root of the thigh into two branches. The dorsal branch, 

 known as the iliac vein, runs forward in the dorsal side of the 

 posterior part of the abdominal cavity, and unites with the 

 sciatic vein to form the renal-portal.. The femoral and sciatic 

 veins are further connected by a small vessel, the ramus com- 

 municans iliacus, which runs transversely from one to the 

 other at the base of the thigh. The second or ventral branch 

 of the femoral vein is known as the pelvic vein ; it passes 

 inwards towards the mid-ventral line of the abdominal wall, 

 and unites, just in front of the pubic cartilages, with its fellow 

 of the opposite side to form the anterior abdominal vein. 

 This is a large, median, unpaired vessel, which receives at its 

 point of origin the vesical vein from the bladder, and then 



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