RADIUS. ULXA. 



81 



articular surface, which is slightly concave, is somewhat five-sided and divided 

 by a line into a quadrilateral internal part, which articulates with the seini- 

 lunar bone, and a triangular external part, which articulates with the 

 scaphoid bone. The ulnar articular surface is vertical, and forms a right 



Fig. 71. RIGHT RADIUS FROM BEFORE. 

 Fig. 72. RIGHT RADIUS FROM BEHIND. 



i 



Fig. 71. 



Fig. 72. 



1, head, showing the hollow above for the humerus, 

 and the short cylindrical surface surrounding it for 

 the ulnar articulation ; 2, the neck ; 3, the tubercle ; 

 4, is opposite to the oblique line ; 5, internal border 

 or interosseous ridge ; the shaded part near 5 marks 

 the slight hollow in which the flexor longus pollicis 

 muscle lies ; 6, carpal articular surface ; 7, styloid 

 process ; 8, the articular hollow for the lower end 

 of the ulna; 9, impression at the middle of the 

 outer border of the attachment of the pronator radii 

 teres ; 10 and 11, oblique depressions marking the 

 places of the extensor longus digitorum and extensor 

 ossis metacarpi pollicis ; between 7 and 8, the dorsal 

 grooves for the tendons of the extensor muscles. 



10 



angle with the inferior surface ; it is concave 

 from before backwards, forming a semilunar 

 cavity, in which the rounded lower end of the 

 ulna plays. At the external angle of the 

 inferior surface a part projecting downwards, 

 called the styloid process, gives attachment to 

 the external lateral ligament of the wrist 

 joint, while the anterior and posterior margins 

 are likewise rough and prominent for other liga- 

 ments. On its external and posterior aspects 

 the inferior extremity of the radius is marked 

 by grooves, which transmit the extensor ten- 

 dons. Thus, on the external border, is a 

 flat groove directed downwards and forwards 

 which lodges the extensor ossis metacarpi and 7 7 



extensor primi internodii pollicis ; and on the 



posterior surface are three grooves, the middle one of which, oblique and 

 narrow, and with prominent borders, lodges the extensor secundi internodii 

 pollicis ; while of the two others, which are broad and shallower, the 

 external, subdivided by a slight mark, gives passage to the extensores carpi- 

 radiales longior and brevior, and the internal transmits the extensor communis 

 digitorum and extensor iudicis. 



THE ULNA. 



The ulna is the internal of the two bones of the forearm. It is longer 

 than the radius by the extent of the olecranon process. It is largest at 

 its upper extremity, and is curved and three-sided elsewhere, except for a 

 short distance at the lower end, where it is straight and cylindrical. It is 

 inclined downwards and outwards from the humerus in such a direction 

 that a straight line passing from the great tuberosity of the humerus down- 

 wards through the capitellum would touch the lower end of the ulna. 



The ulna articulates with the humerus and the radius : in the natural 

 skeleton it is not in contact with the carpal bones, being separated from 

 the cuneiform bone by an interarticular fibro- cartilage. 



