RADIO-ULNAR. 291 



on that bone ; from these points it is reflected over the anterior, posterior, and 

 lateral ligaments ; and forms a pouch between the lesser sigmoid cavity, the 

 internal surface of the annular ligament, and the circumference of the head of 

 the radius. 



The Muscles in relation with the joint are, in front, the Brachialis .Anticus ; 

 behind, the Triceps and Anconeus ; externally, the Supinator Brevis, and the 

 common tendon of origin of the extensor muscles ; internally, the common 

 tendon of origin of the flexor muscles, and the Flexor Carpi Ulnaris, with the 

 ulnar nerve. 



The Arteries supplying the joint are derived from the communicating 

 branches between the superior profunda, inferior profunda, and anastomotio 

 branches of the brachial, with the anterior, posterior, and interosseous recur- 

 rent branches of the ulnar, and the recurrent branch of the radial. These 

 vessels form a complete chain of inosculation around the joint. 



The Nerves are derived from the ulnar, as it passes between the internal 

 condyle and the olecranon ; and a few filaments from the musculo-cutaneous. 



Actions. The elbow is a perfect hinge-joint; its movements are, consequently, 

 limited to flexion and extension, the exact apposition of the articular surfaces 

 preventing the least lateral motion. 



YI. RADIO-ULNAE 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; 

 2. The middle radio-ulnar ; and, 3. The inferior radio-ulnar articulations. 



1. SUPERIOR EADIO-ULNAR ARTICULATION. 



This articulation is a lateral ginglymus. The bones entering into its forma- 

 tion are the inner side of the circumference of the head of the radius, which 

 rotates within the lesser sigmoid cavity of the ulna. These surfaces are covered 

 with cartilage, and invested with a duplicature of sy no vial membrane, continu- 

 ous with that which lines the elbow-joint. Its only ligament is the annular or 

 orbicular. 



The Orbicular Ligament (Fig. 200) 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 three-fourths of a fibrous 

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

 broader at the upper part of its circumference than below, 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. 



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 forwards being called pronation ; 

 rotation backwards, supination. 



2. MIDDLE RADIO-ULNAR ARTICULATION. 



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



ligaments. 



Oblique. Interosseous. 



The Oblique or Round Ligament (Fig. 199) is a small round fibrous cord, 

 which extends obliquely downwards and outwards, 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 



