Metacarpus (Cannon) Veterinary Anatomy

Ox Standard:
- Fused Mc III & IV (Cannon Bone).
- Dorsal longitudinal groove.
- Two distal trochleae for digits.
- Mc V is a vestigial lateral 'button'.

METACARPUS — THE CANNON BONE

The metacarpus is the segment between the carpus and the digits. In ox it is a single fused 'cannon bone' (Mc III + Mc IV), reflecting the cloven-hoof design.

OX METACARPUS:
• Single large bone formed by fusion of 3rd + 4th metacarpals (Mc III + Mc IV) early in fetal life.
• A dorsal longitudinal groove on the front face marks where the two original bones fused — useful in distinguishing metacarpus from metatarsus (groove deeper in metatarsus).
• Distal end bears two trochleae (one for each digit) separated by a notch.
• Each trochlea has a sagittal ridge that fits between the two proximal sesamoids of its digit at the fetlock.
• Mc V = vestigial; often a small 'button' on the latero-proximal aspect.
• Mc II = absent or extremely reduced.
• Internally the bone retains TWO medullary cavities that join at the centre — evidence of fusion.

HOW TO TELL FORELIMB CANNON FROM HINDLIMB CANNON:
• Forelimb (metacarpal) cannon — SHORTER, more rounded in cross-section, dorsal groove shallow.
• Hindlimb (metatarsal) cannon — LONGER, more square / quadrilateral, dorsal groove deeper.

HORSE METACARPUS:
• Mc III = main cannon bone (single weight-bearing column for the single digit).
• Mc II + Mc IV = 'splint bones' — vestigial, taper distally to small 'buttons'; bound to Mc III by interosseous ligament.
• Splint bones don't bear weight but their inflammation = 'splints' (common young horse lameness).

DOG METACARPUS:
• 5 separate metacarpals (full pentadactyl pattern).
• Mc I supports the dewclaw (digit I).
• Mc II-V bear weight on their respective digits.

FOWL METACARPUS:
• Carpometacarpus = fusion of carpal + metacarpal bones into a single rigid bone of the wing.

UG MEMORY: 'Ox = MC III+IV fused = cannon. Horse = only MC III is cannon, MC II + IV = splints. Dog = 5 separate. Fowl = carpometacarpus fused.'