Tendon of the third interosseous muscle [Suspensory ligament]

The tendon of the third interosseous muscle (middle interosseous muscle) is an entirely tendinous muscle called the suspensory ligament (SL).

The suspensory ligament (SL) originates on the proximal aspect of the third metacarpal (cannon) and distal carpal bones. The SL then lies within the groove between the second and fourth metacarpals (splint bones), deep to the superficial digital flexor tendon (SDFT) and deep digital flexor tendon (DDFT). 

Distally, suspensory ligament (SL) bifurcates into two extensor branches, a medial and a lateral one, which insert on the proximal sesamoid bones and extend laterally and medially, either side of the metacarpophalangeal (feltlock) joint, to join the common digital extensor tendon. 

The major function of the SL is as part of the suspensory apparatus that support to the metacarpophalangeal (fetlock) joint by preventing hyperextension and also limits palmar flexion. 

• Origin: Proximal region of third metacarpal(cannon bone) and distal row of carpal bones
• Insertion: Proximal sesamoids, joins common digital extensor tendon
• Action: Prevents fetlock hyperextension and limits palmar flexion

Sources: https://en.wikivet.net/Tendons_-_Horse_Anatomy