EXTENSOR CARPI ULNARIS 391 



In the distal quarter of the forearm the tendons of these two muscles are crossed by the long 

 abductor and the short extensor of the thumb. Beneath the dorsal carpal ligament the tendon 

 of the short radial extensor is crossed by the tendon of the long extensor of the thumb. 



Variations. — The tendon often sends no slip to the second metacarpal. Fusion of the two 

 radial extensors is frequent. The fused muscle may have from one to four tendons. The 

 extensor carpi radialis intermedius of Wood is a muscle which arises, rarely directly from the 

 humerus, but not infrequently as a slip from one or both radial extensors. It is inserted into 

 the second or third metacarpal bone or into both. The extensor carpi radialis accessorius is a 

 muscle which has an origin like the extensor intermedius, but which terminates on the base of 

 the metacarpal or first phalanx of the thumb, the short abductor of the thumb, or some neigh- 

 bouring structure. 



The extensor digitorum communis (figs. 367, 368). — Origin. — From a tendon attached to 

 the lateral epicondyle, and from intermuscular septa which lie between the head of the muscle 

 and the short radial extensor, the extensor of the little finger, and the supinator muscle. 



Insertion. — By four tendons into the bases of the phalanges of the fingers. 



Structure. — The fibre-bundles arise from the interior of the pyramidal case formed by the 

 tendon, the fascia, and intermuscular septa, and pass distally to converge on four tendons which 

 begin in the middle of the forearm, become free a little above the wrist, pass under the dorsal 

 carpal ligament in a groove common to them and the tendon of the extensor indicis proprius, and 

 diverge to the backs of the fingers. Opposite the metacarpo-phalangeal joint each tendon gives 

 rise on its under surface to a band which becomes attached to the base of the first phalanx of its 

 respective digit. The tendon is also closely bound to the joint by fibrous bands connected 

 with the palmar fascia. On the dorsum of the first phalanx the tendon expands and is bound 

 to an aponeurotic extension from the interosseous and lumbrical muscles. The tendon divides 

 into three bands. The middle band passes to the base of the second phalanx, the lateral bands 

 pass laterally around the joint to be inserted into the back of the base of the third phalanx. 

 The lateral bands are bound to the second joint by a thin layer of transverse and oblique fibres. 



An obliquely transverse band usually passes from the tendon of the index to that of the 

 middle finger above the heads of the metacarpals. The tendon to the index finger is united to 

 the tendon of the extensor indicis proprius opposite the metacarpo-phalangeal articulation. 

 The tendon to the ring finger usually sends a slip to join the tendon of the middle finger. The 

 fourth tendon lies near that of the ring finger and divides into two sHps, one of which joins the 

 tendon of the ring finger and one goes to the little finger to join the tendon of the extensor digiti 

 quinti proprius. 



Nerve-supply .—From a branch which arises from the deep radial (posterior interosseous) 

 nerve as it emerges from the supinator (brevis) muscle. From this several twigs enter the deep 

 surface of the middle third of the beUy. Often the nerve is bound up with the nerve to the 

 extensor of the little finger and the ulnar extensor. On the other hand, there may be several 

 separate branches to the muscle. The nerve fibres arise from the sixth, seventh, and eighth 

 cervical nerves. 



Action. — The muscle extends the two terminal phalanges on the basal, the basal on the 

 metacarpus, and the hand at the wrist. The extensor action is strongest on the first phalanx. 

 The cross-bands between the tendons hinder the independent extension of the middle and ring 

 fingers, while the special extensors of the index and little fingers makes the movements of these 

 fingers freer. When the hand is abducted toward the radial side, the extensor muscles tend to 

 draw the fingers ulnarward. When the hand is abducted toward the ulnar side, the muscles 

 tend to draw the fingers toward the thumb. When the hand is in the mid-position the ring 

 finger and little finger are abducted and the index-finger is adducted. (Frohse.) 



Relations. — It is superficially placed. Under it lie the deep muscles of the back of the 

 forearm, the interosseous vessels, and the deep radial (posterior interosseous) nerve. It lies 

 between the short radial carpal extensor and the extensor of the little finger. 



Variations. — There is considerable variation in the extent of isolation of the parts going 

 to the various fingers. That to the index-finger is the one most frequently isolated. At times 

 the tendon to the index or Little finger may be wanting. More frequently one or more of the 

 tendons subdivides to be attached to two or more fingers or to the thumb. The connections 

 between the tendons on the back of the hand vary greatly. 



The extensor digiti quinti proprius (extensor minimi digiti) (figs. 367, 368). — Origin. — 

 Chiefly from the septum between it and the common extensor, but also in part from the septum 

 between it and the extensor ulnaris and from the overlying fascia. 



Structure and insertion. — The fibre-bundles descend in a narrow band which begins near 

 the neck of the radius. They are inserted into the side of a tendon which begins high on the 

 ulnar margin of the muscle. The most distal fibre-bundles extend nearly to the wrist-joint. 

 The tendon passes through the fifth compartment beneath the dorsal carpal ligament, and 

 extends on the back of the fifth metacarpal to the base of the first phalanx of the little finger, 

 where it is joined by a slip from the fourth tendon of the common extensor. The insertion of 

 the tendon is Uke that of the tendons of the common extensor. 



Nerve-supply. — By a branch or branches from the deep radial (posterior interosseous) nerve. 

 The nerve filaments enter the middle third of the fleshy portion of the muscle on its deep 

 surface. The innervation of this muscle is intimately related to that of the preceding. 



Action. — It acts as a portion of the common extensor, but, owing to its separation, in- 

 dependent movement of the little finger is possible. 



Relations. — It Ues between the common extensor and the ulnar extensor and upon the deep 

 muscles of the back of the forearm. 



Variations. — Absence is not very frequent; blending with the common extensor is frequent. 

 Its tendon often divides into two or more slips. The belly may also be doubled. It may have 

 a supplementary origin from the ulna. A tendon slip to the ring-finger is frequently found. 



The extensor carpi ulnaris (figs. 367, 368). — Origin. — By two heads: one from the inferior 

 dorsal portion of the epicondyle by an aponeurotic band attached below the tendon of the 



