POSTERIOR BRACHIAL REGION. 391 



Triceps. It arises by a separate tendon from the back part of the outer con- 

 dyle 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. 



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 BRACHIAL REGION. (Fig. 242.) 

 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 the ridge on the ulna, which runs obliquely down- 

 wards from the posterior extremity of the lesser sigmoid cavity, from the 

 triangular depression in front of it, and from a tendinous expansion which 

 covers the surface of the muscle. 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, and is 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 of the radius, as low down as 

 the insertion of the Pronator Radii Teres. This muscle is pierced by the pos- 

 terior 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 : it lies 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 insertion 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 ; and with the branches of 

 the posterior interosseous artery and nerve which cross it. By its deep surface, 

 with the ulna, interosseous membrane, radius, the tendons of the Extensor 

 Carpi Radialis Longior and Brevior, which it crosses obliquely ; and, at the outer 

 side of the wrist, with the radial vessels. By its upper border, with the Supi- 

 nator 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 inter- 

 osseous membrane. Its direction is similar to that of the Extensor Ossis Meta- 

 carpi, 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 Meta- 



