_23_, Internal head of triceps; _30_, flexor carpi medius; _31_, flexor carpi internus; _31′_, tendon of _31_; _32_, superficial digital flexor; _32′_, radial check ligament; _32″_, tendon of superficial flexor; _33a_, _33b_, _33c_, humeral heads of deep flexor; _33′_, deep flexor tendon; _33″_, subcarpal check ligament; _34_, ulnar head of deep flexor; _35_, radial head of deep flexor; _36_, suspensory ligament. (After Ellenberger, in Leisering’s Atlas.) ] 3. =Flexor carpi externus= (M. extensor carpi ulnaris[61] s. ulnaris lateralis; external flexor of the metacarpus).—This muscle lies on the outer face of the forearm, behind the lateral extensor of the digit. _Origin._—The extensor (external) epicondyle of the humerus, behind and below the lateral ligament. _Insertion._—(1) The outer surface and upper edge of the accessory carpal bone; (2) the proximal extremity of the outer (fourth) metacarpal bone. _Action._—To flex the carpal joint and to extend the elbow. _Structure._—The belly of the muscle is flattened and is intersected by a good deal of tendinous tissue. There are two tendons of insertion. The short tendon is inserted into the accessory carpal bone. The long tendon is detached just above the carpus; it is smaller and rounded; it passes downward and a little forward through a groove on the outer surface of the accessory carpal bone, enveloped by a =synovial sheath=, to reach its insertion on the outer metacarpal bone. A synovial pouch lies under the origin of the muscle at the elbow joint, with the cavity of which it communicates. [Illustration: FIG. 194.—CROSS-SECTION OF PROXIMAL PART OF LEFT CARPUS OF HORSE. The tendons have been removed to show the canals in which they lie. The joint cavity is black. By an oversight the carpal canal for the two flexor tendons behind the posterior ligament of the carpus is not marked. _Cr_, Radial, _Ci_, intermediate, _Cu_, ulnar, _Ca_, accessory carpal bones. ] _Relations._—Superficially, the skin, fascia, and cutaneous branches of the ulnar nerve; deeply, the elbow joint, the ulna, and the flexors of the digit; in front, the lateral extensor of the digit; behind, the middle flexor of the carpus, the ulnar head of the deep flexor, and the ulnar vessels and nerve. _Blood-supply_.—Interosseous, ulnar, and median arteries. _Nerve-supply._—Radial nerve. 4. =Superficial digital flexor= or =flexor perforatus= (M. flexor digitalis sublimis; superficial flexor of the phalanges).—This muscle is situated in the middle of the flexor group, chiefly under cover of the middle flexor of the carpus. _Origin._—(1) The flexor epicondyle of the humerus, between the flexor carpi internus and the deep flexor of the digit; (2) a ridge on the posterior surface of the radius, below its middle and near the internal border.