POSTERIOR BRACHIAL REGION. 313 



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. 



POSTERIOR BKACHIAL REGION. 



Deep Layer. 



Supinator Brevis. Extensor Primi Internodii Pollicis. 



Extensor Ossis Metacarpi Pollicis. Extensor Secundi Internodii Pollicis. 



Extensor Indicis. 



The Supinator Brevis is a broad muscle, of a hollow cylindrical form, curved 

 round the upper third of the radius. It arises from the external condyle of the 

 humerus, from the external lateral ligament of the elbow-joint, and the orbicular 

 ligament of the radius, from an oblique ridge on the ulna, extending down from 

 the posterior extremity of the lesser sigmoid cavity, and from the triangular 

 depression in front of it ; and it also arises from a tendinous expansion which covers 

 its surface. The muscle surrounds the upper part of the radius ; the upper fibres 

 forming a sling-like fasciculus, which encircles the neck of the radius above the 

 tuberosity, to be attached to the back part of its inner surface ; the middle fibres 

 are attached to the outer edge of the bicipital tuberosity ; the lower fibres to the 

 oblique line, as low down as the insertion of the Pronator radii teres. This 

 muscle is pierced by the posterior interosseous nerve. 



Relations. By its superficial surface, with the superficial Extensor and Supi- 

 nator muscles, and the radial vessels and nerve. By its deep surface, with the 

 elbow-joint, the interosseous membrane, and the radius. 



The Extensor Ossis Metacarpi Pollicis is the most external and the largest of 

 the deep Extensor muscles, lying immediately below the Supinator brevis, with 

 which it is sometimes united. It arises from the posterior surface of the shaft of 

 the ulna below the origin of the Anconeus, from the interosseous ligament, and 

 from the middle third of the posterior surface of the shaft of the radius. Passing 

 obliquely downwards and outwards, it terminates in a tendon which runs through 

 a groove on the outer side of the styloid process of the radius, accompanied by 

 the tendon of the Extensor primi internodii pollicis, and is inserted into the base 

 of the metacarpal bone of the thumb. 



Relations. By its superficial surface, with the Extensor communis digitorum, 

 Extensor minimi digiti, and fascia of the forearm ; being crossed by the branches 

 of the posterior interosseous artery and nerve. By its deep surface, with the ulna, 

 interosseous membrane, radius, the tendons of the Extensor carpi radialis longior 

 and brevior ; and, at the outer side of the wrist, with the radial vessels. By its 

 upper border, with the Supinator brevis. By its lower border, with the Extensor 

 primi internodii pollicis. 



The Extensor Primi Internodii Pollicis, the smallest muscle of this group, lies 

 on the inner side of the preceding. It arises from the posterior surface of the 

 shaft of the radius, below the Extensor ossis metacarpi, and from the interosseous 

 membrane. Its direction is similar to that of the Extensor ossis metacarpi, its 

 tendon passing through the same groove on the outer side of the styloid process, 

 to be inserted into the base of the first phalanx of the thumb. 



Relations. The same as those of the Extensor ossis metacarpi pollicis. 



The Extensor Secundi Internodii Pollicis is much larger than the preceding 

 muscle, the origin of which it partly covers in. It arises from the posterior surface 

 of the shaft of the ulna, below the origin of the Extensor ossis metacarpi pollicis, 

 and from the interosseous membrane. It terminates in a tendon which passes 

 through a separate compartment in the annular ligament, lying in a narrow oblique 

 groove at the back part of the lower end of the radius. It then crosses obliquely 

 the Extensor tendons of the carpus, being separated from the other Extensor 

 tendons of the thumb by a triangular interval, in which the radial artery is found ; 

 and is finally inserted into the base of the last phalanx of the thumb. 



