312 MUSCLES AND FASCIAE. 







POSTERIOR BRACHIAL EEGIOX. 

 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 annular 

 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, opposite its corresponding metacarpo-phalangeal articiila- 

 tion, becomes narrow and thickened, gives off a thin fasciculus upon each side of 

 the joint, and 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 ten- 

 dons of the Interossei and Lumbricales. Opposite the first phalargeal joint, this 

 aponeurosis divides into three slips, a middle, and two lateral ; the former is in- 

 serted 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 upper surface of the last phalanx. 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, posterior 

 interosseous vessels and nerve, the wrist-joint, carpus, metacarpus, and phalanges. 

 By its radial border, with the Extensor carpi radialis brevior. 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 compartment in 

 the annular ligament behind the inferior radio-ulnar joint, subdivides into two 

 AS it crosses the hand, and, at the metacarpo-phalangeal articulation, unites with 

 the tendon derived from the long 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 extensor 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 An- 

 coneus, and from the fascia of the forearm. This muscle terminates 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 Triceps. It 

 arises by a separate tendon from the back part of the outer condyle of the humerus; 

 and is inserted into the side of the olecranon, and upper third of the posterior 

 surface of the shaft of the ulna; its fibres diverge from their origin, the upper 

 ones being directed transversely, the lower obliquely inwards. 



