Complex hinge joint.

Ox Standard (6 Bones):
- Proximal: Radial, Intermediate, Ulnar, Accessory.
- Distal: C1 Absent, C2+C3 Fused, C4 present.

CARPUS — THE 'KNEE' OF THE HORSEMAN

The carpus is the equivalent of the human wrist — but in equids it is colloquially called the 'knee' (NOT to be confused with the stifle, which is the true hindlimb knee). It is a complex composite hinge joint between the forearm and metacarpus.

OX CARPUS — 6 BONES IN 2 ROWS:
Proximal Row (4 bones, lateral → medial):
• Accessory Carpal (Cu) — projects backward like a horn; insertion of flexor carpi ulnaris and ulnaris lateralis. Easily palpable.
• Ulnar Carpal (Cui) — articulates with ulna proximally.
• Intermediate Carpal (Ci) — between radius and ulna.
• Radial Carpal (Cr) — articulates with radius proximally.

Distal Row (2 bones in ox):
• C1 (1st carpal) — ABSENT in OX.
• C2+C3 (FUSED) — single bone — characteristic of ruminants.
• C4 (4th carpal) — separate, lateral.

JOINT LEVELS (3 in ox):
1. Antebrachiocarpal joint — between forearm and proximal row → highest mobility.
2. Mid-carpal joint — between rows.
3. Carpo-metacarpal joint — between distal row and metacarpus → little movement.

LIGAMENTS:
• Medial + Lateral Collateral ligaments.
• Multiple short interosseous ligaments connect adjacent bones.
• Palmar carpal ligament + flexor retinaculum hold flexor tendons; together with the accessory carpal bone they form the CARPAL CANAL on the palmar aspect — through it pass the median nerve, median artery, deep digital flexor tendon and superficial digital flexor tendon.

SYNOVIAL STRUCTURES:
• 3 separate joint capsules (one per joint level) usually.
• Tendon sheaths around flexor tendons.
• Subcutaneous bursa over cranial carpus → 'precarpal hygroma' in chronic 'kneeling' cattle.

UG MEMORY: 'Ox = 6 (proximal: 4 + distal: 2 — C1 lost, C2+3 fused). Horse = 7-8 (full row except C1 inconstant). Dog = 7 (Cr + Ci fused into intermedioradial). Pig = 8 (full primitive).'