ELBOW-JOINT 



323 



The portion of the joint between the ulna and humerus is a simple hinge-joint, and allows of 

 movements of flexion and extension only. Owing to the obliquity of the trochlea of the humerus, 

 this movement does not take place in the antero-posterior plane of the body of the humerus. 

 When the forearm is extended and supinated, the axes of the arm and forearm are not in the same 

 line; the arm forms an obtuse angle with the forearm, the hand and forearm being directed lateral- 

 ward. During flexion, however, the forearm and the hand tend to approach the middle line of 

 the body, and thus enable the hand to be easily carried to the face. The accurate adaptation 

 of the trochlea of the humerus, with its prominences and depressions, to the semilunar notch of 

 the ulna, prevents any lateral movement. Flexion is produced by the action of the Biceps brachii 

 and Brachialis, assisted by the Brachioradialis and the muscles arising from the medial condyle 

 of the humerus; extension, by the Triceps brachii and Anconaeus, assisted by the Extensors of 

 the wrist, the Extensor digitorum communis, and the Extensor digiti quinti proprius. 



FIG. 331. Capsule of elbow-joint (distended). 

 Anterior aspect. 



FIG. 332. Capsule of elbow-joint (distended). 

 Posterior aspect. 



The joint between the head of the radius and the capitulum of the humerus is an arthrodial 

 joint. The bony surfaces would of themselves constitute an enarthrosis and allow of movement 

 in all directions, were it not for the annular ligament, by which the head of the radius is bound 

 to the radial notch of the ulna, and which prevents any separation of the two bones laterally. 

 It is to the same ligament that the head of the radius owes its security from dislocation, which 

 would otherwise tend to occur, from the shallowness of the cup-like surface on the head of the 

 radius. In fact, but for this ligament, the tendon of the Biceps brachii would be liable to pull 

 the head of the radius out of the joint. The head of the radius is not in complete contact with 

 the capitulum of the humerus in all positions of the joint. The capitulum occupies only the 

 anterior and inferior surfaces of the lower end of the humerus, so that in complete extension a 

 part of the radial head can be plainly felt projecting at the back of the articulation. In full 

 flexion the movement of the radial head is hampered by the compression of the surrounding soft 

 parts, so that the freest rotatory movement of the radius on the humerus (pronation and supina- 

 tion) takes place in semiflexion, in which position the two articular surfaces are in most intimate 





