Origin ([Fig. 81], m).—The distal surface of the lateral supracondyloid ridge dorsad of the origin of the extensor carpi radialis brevis.
At the junction of the middle and distal thirds of the radius the muscle passes into a large, flat tendon ([Fig. 75], m′) which passes through the groove on the middle of the dorsal surface of the distal end of the radius. The groove is converted into a canal by a strong transverse ligament. The tendon divides near the groove into four portions ([Fig. 84], a) which diverge and pass to the four ulnar digits.
Insertions.—Each division of the tendon as it passes over the dorsal surface of the first phalanx has its border connected by strong fascia to the dorsal surface of the phalanx. It finally passes through the groove at the distal end of the first phalanx and is inserted mostly into the base of the second phalanx; a portion continues distad to be inserted into the third phalanx.
Relations.—Outer surface with the integument and at the origin with the caput laterale ([Fig. 75], h) of the triceps. Radial surface with the extensor longus (l) and extensor brevis. Ulnar surface with the extensor digitorum lateralis (n). Inner surface with the supinator ([Fig. 85], b) and the extensor pollicis brevis ([Fig. 85], a).
Action.—Extensor of the four ulnar digits.
M. extensor digitorum lateralis ([Fig. 75], n). (This corresponds in position to the extensor digiti quinti proprius of man.)
Origin ([Fig. 81], n) from the lateral supracondyloid ridge of the humerus distad of the origin of the extensor communis.
The muscle passes along the ulnar border and dorsal surface of the forearm between the extensor communis ([Fig. 75], m) and the extensor carpi ulnaris (o). Near the carpus it ends in a tendon (n′), which divides immediately into three or four ([Fig. 84], b). These pass through the groove on the ulnar side of the dorsal surface of the radius and diverge to the digits lying beneath the extensor communis tendon.
Insertion ([Fig. 84], b).—The three tendons on the ulnar side join the ulnar sides of the tendons of the extensor communis (a) at their insertions. The radial of the four tendons (not always present) joins the tendon of the extensor indicis (c) and may also give a branch to the radial side of the base of the first phalanx of the third digit.
Relations.—Outer surface with the integument. Radial border with the extensor communis digitorum (m); ulnar border with the extensor carpi ulnaris (o). Inner surface with the supinator ([Fig. 85], b) and the extensor brevis pollicis ([Fig. 85], a); distad with the extensor indicis ([Fig. 85], c).