Abaxial palmar ligament of the proximal interphalangeal joint

In horses, the palmar ligaments of the proximal interphalangeal joint (pastern joint) consists of two central pairs of ligaments (two axial ligaments (medial and lateral) and two abaxial ligaments (medial and lateral)), which run parallel to the straight sesamoidean ligament, and the lateral and medial palmar ligaments:

• The medial and lateral axial palmar ligaments originate from mid proximal phalanx (P1), just axial to the medial and lateral branches of the SDFT and inserts on proximal middle phalanx (P2). Then they blend with the fibers of insertion of the superficial digital flexor tendon (SDFT) at their insertion on proximal middle phalanx (P2). 
• The medial and lateral abaxial palmar ligaments originate medially and laterally, respectively, to the medial and lateral branches of the superficial digital flexor tendon (SDFT) on mid proximal phalanx (P1) and insert on proximal palmar P2 and its fibrocartilage. In ultrasound, they appear as a slightly flattened triangular structures which originate just lateral or medial to the superficial branch and just palmar to the lateral or medial collateral of the proximal interphalangeal (PIP) joint. They tend to be much more prominent in appearance when injured.

Sources: Ultrasound of the Pastern Region - VetFolio