390 MUSCLES AND FASCIA. 



POSTEEIOR BRACHIAL KEGIOX. (Fig. 241.) 

 Superficial Layer. 



Extensor Communis Digitorum. Extensor Carpi Ulnaris. 



Extensor Minimi Digiti. Anconeus. 



The Extensor Communis Digitorum is situated at the back part of the forearm. 

 It arises from the external condyle of the humerus by the common tendon, from 

 the deep fascia, and the intermuscular septa between it and the adjacent muscles. 

 Just below the middle of the forearm it divides into three tendons, which pass, 

 together with the Extensor Indicis, through a separate compartment of the an- 

 nular ligament, lubricated by a synovial membrane. The tendons then diverge, 

 the innermost one dividing into two ; and all, after passing across the back of 

 the hand, are inserted into the second and third phalanges of the fingers in the 

 following manner : Each tendon becomes narrow and thickened opposite the 

 metacarpo-phalangeal articulation, and gives off a thin fasciculus upon each 

 side of the joint, which serves as the posterior ligament ; after having passed 

 the joint, it spreads out into a broad aponeurosis, which covers the whole of 

 the dorsal surface of the first phalanx ; being reinforced, in this situation, by 

 the tendons of the Interossei and Lumbricales. Opposite the first phalangeal 

 joint, this aponeurosis divides into three slips, a middle, and two lateral ; the 

 former is inserted into the base of the second phalanx ; and the two lateral, 

 which are continued onwards along the sides of the second phalanx, unite by 

 their contiguous margins, and are inserted into the dorsal surface of the last 

 phalanx. As the tendons cross the phalangeal joints, they furnish them with 

 posterior ligaments. The tendons of the middle, ring, and little fingers are 

 connected together, as they cross the hand, by small oblique tendinous slips. 

 The tendons of the index and little fingers also receive, before their division, 

 the special extensor tendons belonging to them. 



Relations. By its superficial surface, with the fascia of the forearm and hand, 

 the posterior annular ligament, and integument. By its deep surface, with the 

 Supinator Brevis, the extensor muscles of the thumb and index finger, the 

 posterior interosseous vessels and nerve, the wrist-joint, carpus, metacarpus, 

 and phalanges. By its radial border, with the Extensor Carpi Eadialis Bre- 

 vior. By its ulnar border, with the Extensor Minimi Digiti, and Extensor 

 Carpi Ulnaris. 



The Extensor Minimi Digiti is a slender muscle, placed on the inner side of 

 the Extensor Communis, with which it is generally connected. It arises from 

 the common tendon by a thin tendinous slip ; and from the intermuscular septa 

 between it and the adjacent muscles. Its tendon runs through a separate com- 

 partment in the annular ligament behind the inferior radio-ulnar joint, subdi- 

 vides into two as it crosses the hand, and at the metacarpo-phalangeal articula- 

 tion, unites with the tendon derived from the common Extensor. The common 

 tendon then spreads into a broad aponeurosis, which is inserted into the second 

 and third phalanges of the little finger in a similar manner to the common ex- 

 tensor tendons of the other fingers. 



The Extensor Carpi Ulnaris is the most superficial muscle on the ulnar side 

 of the forearm. It arises from the external condyle of the humerus, by the 

 common tendon ; from the middle third of the posterior border of the ulna 

 below the Anconeus, and from the fascia of the forearm. This muscle termi- 

 nates in a tendon, which runs through a groove behind the styloid process of 

 the ulna, passes through a separate compartment in the annular ligament, and 

 is inserted into the base of the metacarpal bone of the little finger. 



Relations. By its superficial surface, with the fascia of the forearm. By its 

 deep surface, with the ulna, and the muscles of the deep layer. 



The Anconeus is a small triangular muscle, placed behind and below the 

 elbow-joint, and appears to be a continuation of the external portion of the 



