RADIO-ULNAR ARTICULATIONS. 259 



the base of the condyles, that of the ulna and radius should be just below the level of the 

 lesser sigmoid cavity of the ulna and the neck of the radius. In this operation the object is 

 to obtain such union as shall allow free motion of the bones of the forearm ; and, therefore, 

 passive motion must be commenced early, that is to say, about the tenth day. 



VI. Radio-ulnar Articulations. 



The articulation of the radius with the ulna is effected by ligaments which 

 connect together both extremities as well as the shafts of these bones. They may, 

 consequently, be subdivided into three sets : 1, the superior radio-ulnar, which is 

 a portion of the elbow-joint; 2, the middle radio-ulnar; and, 3, the inferior radio- 

 ulnar articulations. 



1. SUPERIOR RADIO-ULNAR ARTICULATION. 



This articulation is a trochoid or pivot-joint. The bones entering into its 

 formation are the inner side of the circumference of the head of the radius rotating 

 within the lesser sigmoid cavity of the ulna. Its only ligament is the annular or 

 orbicular. 



The Orbicular Ligament (Fig. 172) is a strong, flat band of ligamentous fibres, 

 which surrounds the head of the radius, and retains it in firm connection with the 

 lesser sigmoid cavity of the ulna. It forms about four-fifths of an osseo-fibrous ring, 

 attached by each end to the extremities of the lesser sigmoid cavity, and is smaller 

 at the lower part of its circumference than above, by which means the head of the 

 radius is more securely held in its position. Its outer surface, is strengthened 

 by the external lateral ligament of the elbow, and affords origin to part of the 

 Supinator brevis muscle. Its inner surface is smooth, and lined by synovial 

 membrane. The synovial membrane is continuous with that which lines the 

 elbow-joint. 



Actions. The movement which takes place in this articulation is limited to 

 rotation of the head of the radius within the orbicular ligament, and upon the 

 lesser sigmoid cavity of the ulna, rotation forward being called pronation ; rotation 

 backward, supination. Supination is performed by the Biceps and Supinator 

 brevis, assisted to a slight extent by the Extensor muscles of the thumb and, in 

 certain positions, by the Supinator longus. Pronation is performed by the Pro- 

 nator radii teres and the Pronator quadratus, assisted, in some positions, by the 

 Supinator longus. 



Surface Form. The position of the superior radio-ulnar joint is marked on the surface of 

 the body by the little dimple on the back of the elbow which indicates the position of the head 

 of the radius. 



Surgical Anatomy. Dislocation of the head of the radius alone is not an uncommon 

 accident, and occurs most frequently in young persons from falls on the hand when the forearm 

 is extended and supinated, the head of the bone being displaced forward. It is attended by 

 rupture of the orbicular ligament. Occasionally a peculiar injury, which is supposed to be a 

 subluxation, occurs in young children in lifting them from the ground by the hand or forearm. 

 It is believed that the head of the radius is displaced downward in the orbicular ligament, the 

 upper border of which becomes folded over the head of the radius, between it and the capitel- 

 lum of the humerus. 



2. MIDDLE RADIO-ULNAR UNION. 



The interval between the shafts of the radius and ulna is occupied by two 

 ligaments. 



Oblique. Interosseous. 



The Oblique or Round Ligament (Fig. 171) is a small, flattened fibrous band 

 Avhich extends obliquely downward and outward from the tubercle of the ulna at 

 the base of the coronoid process to the radius a little below the bicipital tuberosity. 

 Its fibres run in the opposite direction to those of the interosseous ligament, and 

 it appears to be placed as a substitute for it in the upper part of the interosseous 

 interval. This ligament is sometimes wanting. 



