THE BONES OF THE UPPER LIMB 195 



wards as a stout conical process, terminating in a round tip which 

 gives attachment to the external lateral ligament of the wrist-joint. 

 The inner surface is covered by the cartilage of the carpal articular 

 surface. The outer surface presents a groove directed downwards 

 and slightly forwards, and subdivided into two compartments, the 

 outer of which transmits the tendon of the extensor ossis metacarpi 

 poUicis, and the inner that of the extensor bre\as polUcis. This 

 groove is separated from the pronator surface in front by a promi- 

 nent ridge which gives attachment to the posterior annular ligament. 

 The anterior surface supports a portion of the radial artery. 



The sigmoid cavity is concave from before backwards, and articu- 

 lates vdth the outer convex surface of the head of the ulna. 



The carpal articular surface is of large size, and its plane is 

 oblique, being sloped outwards and a little downwards. It is 

 concave from before backwards, and from side to side, and is 

 divided into two parts by an ant€ro- posterior elevation. The 

 outer division is triangular, its cartilage being prolonged on to 

 the inner surface of the styloid process, and it articulates with 

 the scaphoid bone. The inner division is quadrilateral, and 

 articulates with the semilunar bone. It is separated from the 

 sigmoid cavity by a sharp concave margin, which gives attach- 

 ment to the base of the triangular interarticular fibro-cartilage. 

 Immediately above the anterior border there is a rough sur- 

 face for the attachment of the anterior ligament of the wrist - 

 joint. The posterior border is on a slightly lower level than the 

 anterior, and is irregularly convex. It presents about its centre 

 a prominent elevation, called the radial tubercle, and is divided 

 into three grooves — outer, middle, and inner. The outer groove, 

 which is broad, is bounded externally by a ridge which separates 

 it from the groove on the outer surface of the styloid process, 

 and internally by the radial tubercle. It is usually subdivided 

 by a faint line into two compartments, the outer of which transmits 

 the tendon of the extensor carpi radialis longior, and the inner 

 that of the extensor carpi radialis brevior. The middle groove, 

 narrow and deep, is directed from above downwards and outwards, 

 and is boimded externally by the radial tubercle, which slightly 

 overhangs it, and internally by an oblique ridge separating it from 

 the inner groove. It transmits the tendon of the extensor longus 

 pollicis (secimdi intemodii poUicis). The inner groove is single, 

 and transmits the tendons of the extensor communis digitorum 

 and extensor indicis. It is separated from the sigmoid cavity by 

 a sharp ridge which, with the ulna in position, bounds a groove for 

 the tendon of the extensor minimi digiti. The ridges separating 

 the grooves give attachment to deep expansions of the posterior 

 annular ligament, which, with the ligament, convert the grooves 

 into fibro-osseous canals. 



The lower extremity presents several nutrient foramina for 

 branches of the anterior and posterior interosseous cirteries, and 

 anterior and posterior carpal arches. 



