THE POSTERIOR RADIO- ULNA R REGION. 397 



the slip to the second, and receives a very considerable part of the fourth tendon; 

 the fourth, or innermost tendon, divides into two parts : one goes to join the 

 third tendon, the other, reinforced by the Extensor minimi digiti, goes to the 

 little finger. Each tendon opposite the metacarpo-phalangeal articulation becomes 

 narrow and thickened, and gives off a thin fasciculus upon each side of the joint, 

 which blends with the lateral ligaments and 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 onward 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. 



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 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, then 

 divides into two as it crosses the hand, the outermost division being joined by the 

 slip from the innermost tendon of the common extensor. The two slips thus formed 

 spread into a broad aponeurosis, which after receiving a slip from the Abductor 

 minimi digiti is inserted into the second and third phalanges. The tendon is 

 situated on the ulnar side of, and somewhat more superficial than, the common 

 extensor. 



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 ; by an aponeurosis from the posterior border of the ulna in common 

 with the Flexor carpi ulnaris and the Flexor profundus digitorum ; and from the 

 deep 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 compart- 

 ment in the annular ligament, and is inserted into the prominent tubercle on the 

 ulnar side of the base of the metacarpal bone of the little finger. 



Relations. By its superficial surface, with the deep 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 fourth 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 inward. 



Relations. By its superficial surface, with a strong fascia derived from the 

 Triceps ; by its deep surface, with the elbow-joint, the orbicular ligament, the 

 ulna, and a small portion of the Supinator brevis. 



Deep Layer (Fig. 237). 



Supinator brevis. Extensor brevis pollicis. 



Extensor ossis metacarpi pollicis. Extensor longus pollicis. 



Extensor indicis. 



