THE POSTERIOR VENA CAVA 595 



The dvcp veins of the forearm are variable. Commonly two posterior radial 

 veins (^"enie medianoradiales) accomjiany the artery of like name, one in front 

 and one behind. A third vein arises by radicles emerging from the proximal part 

 of the deep flexor; it joins the posterior satellite of the posterior radial artery 

 or forms one of the roots of the brachial vein. The common interosseous vein 

 joins the posterior satellite. The anterior radial vein (\. collateralis radialis) is 

 a satellite of the artery. The ulnar vein is usually doul)le at its proximal part, 

 and communicates with the deep Ijrachial vein. 



There are three chief metacarpal veins. The internal metacarpal vein (V. 

 metacarpea volaris superficialis medialis) arises from the volar venous arch above 

 the fetlock. It is the largest vein of the region and lies in front of the large meta- 

 carpal artery. It separates from the artery at the proximal end of the metacarpus, 

 passes upward on the inner part of the posterior surface of the carpus, under cover 

 of the superficial layer of the posterior annular ligament, and is continued as the 

 ce])halic vein. It communicates at its proximal part with the origin of the posterior 

 radial veins liy a short but relatively large l)ranch. The external metacarpal vein 

 (\'. metacarpea volaris superficialis lateralis) arises from the venous arch above the 

 fetlock and passes upward behind the external border of the suspensory ligament 

 in front of the external branch of the median nerve and accompanied by a small 

 artery. At the proximal end of the metacarpus it is connected with the deep meta- 

 carpal vein by two transverse anastomoses which pass across the suspensory liga- 

 ment. It then passes upward as a satellite of the external volar metacarpal artery 

 and concurs in the origin of the ulnar and posterior radial veins. The deep meta- 

 carpal vein (V. metacarpea volaris profunda medialis) arises from the venous arch, 

 passes forward between the two branches of the suspensory ligament, and ascends 

 on the posterior surface of the large metacarpal bone. At the proximal end of the 

 latter it communicates with the other metacarpal veins, ascends with the small 

 metacarpal artery, and concurs in forming the radial veins. 



The venous arch (Arcus venosus volaris), from which the metacarpal veins arise, 

 is situated above the sesamoids of tlie fetlock between the sus]:)ensory ligament 

 and the deep flexor tendon. It is formed by the junction of the two digital veins. 



The digital veins, internal and external (V. digitalis medialis, lateralis), 

 drain the venous plexuses of the foot. They arise at the upper edge of the lateral 

 cartilages and ascend in front of the corresponding arteries. 



It is convenient to recognize the following venous plexuses of the foot, whicli, 

 however, communicate very freely: 



1. The coronary plexus encircles the upper part of the foot. It is attached 

 to the terminal ]^art of the extensor tendon, the lateral cartilages, and the bulbs 

 of the plantar cushion. 



2. The dorsal (or laminal) plexus covers the dorsal or wall surface of the third 

 phalanx in the deep layer of the matrix of the wall of the hoof. It forms the cir- 

 cumflex vein of the third phalanx or vein of the distal border of the third phalanx, 

 which corresponds to the artery of like name. 



3. The volar plexus is in the deep layer of the matrix of the sole of the hoof 

 and on the deep surface of the lateral cartilages. It communicates around th(^ 

 inferior border of the third phalanx with the dorsal plexus and through the lateral 

 cartilages with the coronary plexus. 



The deep vein of the third phalanx accompanies the terminal i)art of the digital 

 artery. It drains the intraosseous plexus. 



THE POSTERIOR VENA CAVA (Figs. 270, 450) 

 The posterior vena cava (V. cava caudalis) returns almost all of the Ijlood from 

 the abdomen, pelvis, and pelvic limbs. It is formed by the confluence of the right 



