138 ARTHROLOGY, OR ANATOMY OF ARTICULAT1O\ ',S 



THE RADIOULNAR ARTICULATION 



It is divided into superior, middle, and inferior radioulnar 

 articulations. 



The Superior Articulation 



This is a trochoid or pivot joint, formed by the inner side 

 of the head of the radius rotating within the lesser sigmoid 

 cavity of the ulna. 



The orbicular ligament surrounds the head of the radius; 

 it forms about |- of an osseofibrous ring and is attached to the 

 lesser sigmoid cavity of the ulna. 



The synovial membrane lines the smooth, inner surface, 

 and is continuous with that which lines the elbow-joint. 



The Middle Radioulnar Articulation 



The oblique ligament (chorda transversalis) is a flatly rounded 

 cord running from the tubercle of the ulna on the coronoid 

 process down and out to a point on the radius a little below 

 the bicipital tuberosity. Its fibers have an opposite direction 

 to those of the interosseous ligament. The oblique may be 

 wanting, or may exist as a tendinous slip to the flexor longus 

 pollicis; it may be double, the upper band passing from the 

 small sigmoid notch and orbicular ligament to a point above 

 the bicipital tuberosity. 



The interosseous ligament (membrane) connects the inter- 

 osseous ridges of the radius and ulna. The fibers pass down 

 and into the ulna in such a direction that if the hand press 

 against resistance the radius would drag the ulna after it. 

 The ligament is divisible here and there into several layers, 

 some fibers coming from the anterior surface of the radius. 

 It is deficient above, commencing on the radius at the insertion 

 of the oblique ligament, leaving a space between the two for 

 the posterior interosseous vessels. Just above 'its lower end 

 is an oblique opening between two layers of the ligament for 

 the passage of the anterior interosseous vessels. The lower 

 edge is almost vertical, the fibers ending higher on the ulna 

 than on the radius and running in a direction opposite to the 



