The =joint capsule= may be regarded, so far as the fibrous part is concerned, as being common to all three joints. It is attached close to the margin of the articular surface of the radius above and the metacarpus below; its deep face is also attached to a considerable extent to the carpal bones and to the small ligaments. Its anterior part, the =dorsal= or =anterior common ligament=, is rather loose, and assists in forming the fibrous canals for the extensor tendons. Its posterior part, the =volar= or =posterior common ligament=, is very thick and dense, and is closely attached to the carpal bones. It levels up the irregularities of the skeleton here, and forms the smooth anterior wall of the carpal canal. It is continued downward to form the =subcarpal= or =inferior check ligament=, which blends with the tendon of the flexor perforans about the middle of the metacarpus. [Illustration: FIG. 146.—FRONTAL SECTION OF CARPAL JOINTS OF HORSE (RIGHT SIDE). _l.u._, External, _l.r._, internal, lateral ligament; _Cr_, radial carpal; _Ci_, intermediate carpal; _Cu_, ulnar carpal; _C2_, second carpal; _C3_, third carpal; _C4_, fourth carpal; _Mc2_, second (internal) metacarpal; _Mc3_, third (large) metacarpal; _Mc4_, fourth (external) metacarpal. ] The =synovial membrane= forms three sacs corresponding to the three joints. The =radio-carpal sac= is the most voluminous; it includes the joints formed by the accessory carpal bone, and also those between the proximal carpal bones as far as the interosseous ligaments. The =intercarpal sac= sends extensions upward and downward between the bones of the two rows as far as the interosseous ligaments; it communicates between the third and fourth carpal bones with the =carpo-metacarpal sac=. The latter is very limited in extent, and is closely applied to the bones; it incloses the carpo-metacarpal joint, and lubricates also the lower parts of the joints between the distal carpal bones and the intermetacarpal joints. The =external lateral ligament= (Ligamentum carpi collaterale ulnare) is attached above to the external tuberosity of the distal end of the radius. Its long superficial part is attached below to the proximal end of the external small metacarpal chiefly, but some fibers end on the large metacarpal bone. A canal for the lateral extensor tendon separates a short deep band which ends on the ulnar carpal bone. Other deep fibers connect the latter with the fourth carpal bone, and the fourth carpal with the metacarpus.