692 BLOOD-VASCULAR SYSTEM OF THE HORSE 
it joins the posterior satellite of the median artery or forms one of the roots of the 
brachial vein. The common interosseous vein joins the posterior satellite. The 
anterior radial vein (V. collateralis radialis) is a satellite of the artery. The ulnar 
vein is usually double at its proximal part, and communicates with the deep brachial 
vein. 
There are three chief metacarpal veins. The medial metacarpal or common 
digital 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 common digital artery. It separates from the artery at the proximal 
end of the metacarpus, passes upward on the medio-volar surface of the carpus, 
under cover of the superficial layer of the volar annular ligament, and is con- 
tinued as the cephalic vein. It communicates at its proximal part with the origin 
of the median veins by a short but relatively large branch. The lateral metacarpal 
vein (V. metacarpea volaris superficialis lateralis) arises from the venous arch above 
the fetlock and passes upward behind the lateral border of the suspensory ligament 
in front of the lateral volar nerve and accompanied by a small artery. At the 
proximal end of the metacarpus it is connected with the deep metacarpal vein by 
two transverse anastomoses which pass across the suspensory ligament. It then 
ascends as a satellite of the lateral volar metacarpal artery and concurs in the 
origin of the ulnar and median veins. The deep metacarpal 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 medial volar metacarpal artery, 
and concurs in forming the median veins. 
The venous arch (Arcus venosus volaris), from which the metacarpal veins 
arise, 1s situated above the sesamoids of the fetlock, between the suspensory liga- 
ment and the deep flexor tendon. It is formed by the junction of the two digital 
veins. 
The digital veins, medial and lateral (V. digitalis medialis, lateralis), drain the 
venous plexuses of the foot. They arise at the proximal edge of the cartilages of 
the third phalanx and ascend in front of the corresponding arteries. 
It is convenient to recognize the following venous plexuses of the foot, which, 
however, communicate very freely: 
1. The coronary plexus encircles the upper part of the foot. It covers the 
terminal part of the extensor tendon, the cartilages of the third phalanx, and the 
bulbs of the digital cushion. 
2. The dorsal (or laminal) plexus covers the dorsal surface of the third phalanx 
in the deep layer of the corium of the wall of the hoof. It forms the circumflex 
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 corium of the sole of the hoof 
and on the deep surface of the cartilages of the third phalanx. It communicates 
around the distal border of the third phalanx with the dorsal plexus and through the 
cartilages with the coronary plexus. 
The deep vein of the third phalanx accompanies the terminal part of the 
digital artery. It drains the intraosseous plexus contained in the third phalanx. 
THE POSTERIOR VENA CAVA (Figs. 554, 575) 
The posterior vena cava (V. cava caudalis) returns almost all of the blood from 
the abdomen, pelvis, and pelvic limbs. It is formed by the confluence of the right 
and left common iliac veins at the fifth lumbar vertebra, above the terminal part 
of the aorta, and chiefly to the right of the median plane. 
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