RADIO-ULNAR JOINTS 261 



The lateral portion, the radial collateral ligament (fig. 294), is attached above 

 to the lower part of the lateral epicondyle, and from this the fibres radiate to 

 their attachment into the lateral side of the annular ligament, a few fibres being 

 prolonged to reach the neck of the radius. The anterior fibres reach further 

 forward than the posterior do behind. It is strong and well-marked, but less 

 so than the medial portion. 



The synovial membrane lines the whole of the capsule, and extends into the 

 superior radio-ulnar joint, lining the annular ligament. 



Outside the synovial membrane, but inside the capsule, are often seen some pads of fatty 

 tissue; one is situated on the medial side at the base of the olecranon, another is seen on the 

 lateral side projecting into the cavity between the radius and ulna; this latter, with a fold of 

 synovial membrane opposite the front of the lateral lip of the trochlea, suggests the divisiori of 

 the joint into two parts — one medially for the ulna, and another laterally for the radius. 

 There are also pads of fatty tissue at the bottom of the olecranon and coronoid fossae, and at 

 the tip of the olecranon process. 



The arterial supply is derived from each of the vessels forming the free anastomosis around 

 the elbow, and there is also a special branch to the front and lateral side of the joint, from the 

 brachial artery, and the arterial branch to the brachialis also feeds the front of the joint. 



The nerve-supply comes chiefly from the musculo-cutaneous; the ulnar, median, and radial 

 (musculo-spiral) also give filaments to the joint. 



Relations. — In front of the joint, and in immediate relation with the capsule, are the 

 brachialis, the superficial and deep branches of the radial (musculo-spiral) nerve, the radial re- 

 current artery, and the brachio-radiaUs. The brachial artery, the median nerve, and the pro- 

 nator teres are separated from the capsule by the brachialis. Directly behind the capsule 

 are the triceps, the anconeus, and the posterior interosseous recurrent artery. On the medial 

 side are the ulnar nerve, the superior ulnar collateral (posterior ulnar recurrent) artery, and the 

 upper parts of the flexor carpi ulnaris and flexor digitorum subhmis. On the lateral side lie 

 the extensor carpi radiaUs longus and the upper part of the common tendon of origin of the 

 superficial extensors of the wrist and fingers. 



The movements permitted at the elbow are those of a true hinge joint, viz., flexion and 

 extension. These movements are oblique, so that the forearm is inclined medially in flexion, 

 and laterally in extension; they are hmited by the contact respectively of the coronoid and ole- 

 cranon processes of the ulna w'ith their corresponding fossse on the humerus, and their extent is 

 determined by the relative proportion between the length of the processes and depth of the 

 fossae which receive them, rather than by the tension of the ligaments, or the bulk of the soft 

 parts over them. The anterior and posterior portions of the capsule, together with the corres- 

 ponding portions of the collateral ligament, are not put on the stretch during flexion and exten- 

 sion; but, although they may assist in checking the velocity, and thus prevent undue force of 

 impact, they do not control or determine the extent of these movements. The limit of exten- 

 sion is reached when the ulna is nearly in a straight line with the humerus; and the limit of 

 flexion when the ulna describes an angle of from 30° to 40° with the humerus. 



The obliquity of these movements is due to the lateral inclination of the upper and back 

 part of the trochlear surface, and the greater prominence of the medial lip of the trochlea below; 

 thus the plane of motion is directed from behind forward and medially, and carries the hand 

 toward the middle third of the clavicle. The obliquity of the joint, the twist of the shaft of 

 the humerus, and the backward direction of its head, all tend to bring the hand toward the mid- 

 line of the body, under the immediate observation of the eye, whether for defence, employment, 

 or nourishment. This is in striking contrast to the lower limb, where the direction of the foot 

 diverges from the median axis of the trunk, thus preventing awkwardness in locomotion. In 

 flexion and extension, the cup-like depression of the radial head ghdes upon the capitulum, and 

 the medial margin of the radial head travels in the groove between the capitulum and the 

 trochlea. This allows the radius to rotate upon the humerus while following the ulna in all 

 its movements. In full extension and supination, the head of the radius is barely m contact 

 with the inferior surface of the capitulum, and projects so much backward that its posterior 

 margin can be felt as a prominence at the back of the elbow. In full flexion the anterior edge 

 of the radial head is received into, and checked against, the depression above the capitulum ; 

 while in mid-flexion the cup-like depression is fairly received upon the capitulum, and m this 

 position, the radius being more completely steadied by the humerus than in any other, pro- 

 nation and supination take place most perfectly. 



Muscles which act upon the elbow-joint. — Flexors. — Brachialis, biceps, brachio-radialis, 

 pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum sublimis, flexor carpi 

 ulnaris. . , , 



Extensors. — Triceps, anconeus, and the muscles which spring from the lateral epico ndyle 



5. THE UNION OF THE RADIUS WITH THE ULNA 



The radius is firmly united to the ulna by two joints, and an intermediate 

 fibrous union, viz.: — . 



(a) The superior radio-ulnar — whereat the head of the radius rotates within 

 the radial notch and annular ligament. 



(6) The union of the shafts — the mid radio-ulnar union. 



(c) The inferior radio-ulnar — whereat the lower end of the radius rolls round 

 the head of the ulna. 



