_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: