THE ELBOW-JOINT. 



257 



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

 and the olecranon ; a filament from the musculo-cutaneous (Riidinger), and two 

 from the median (Macalister). 



Actions. The elbow-joint comprises three different portions viz., the joint 

 between the ulna and humerus, that between the head of the radius and the 

 humerus, and the superior radio-ulnar 

 articulation, described below. All these 

 articular surfaces are invested by a com- 

 mon synovial membrane, and the move- 

 ments of the whole joint should be studied 

 together. The combination of the move- 

 ments of flexion and extension of the 

 forearm with those of pronation and supi- 

 nation of the hand, which is ensured by 

 the two being performed at the same joint, 

 is essential to the accuracy of the various 

 minute movements of the hand. 



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 trochlear surface of the humerus, 

 this movement does not take place in a 

 straight line ; so that when the forearm is 

 extended and supinated the axis of the 

 arm and forearm is not in the same line, 

 but the one portion of the limb forms an 

 angle with the others, and the hand, with 

 the forearm, is directed outward. During 

 flexion, on the other hand, 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 shape 

 of the articular surface of the humerus, 

 with its prominences and depressions accurately adapted to the opposing surfaces 

 of the olecranon, prevents any lateral movement. Flexion is produced by the 

 action of the Biceps and Brachialis anticus, assisted by the muscles arising from 

 the internal condyle of the humerus and the Supinator longus ; extension, by the 

 Triceps and Anconeus, assisted by the extensors of the wrist and by the Extensor 

 communis digitorum and Extensor minimi digiti. 



The joint between the head of the radius and the capitellum or radial head of 

 the humerus is an arthrodial joint. The bony surfaces would of themselves con- 

 stitute an enarthrosis, and allow of movement in all directions were it not for the 

 orbicular ligament by which the head of the radius is bound down firmly to the 

 sigmoid cavity 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 constantly occur as a consequence of the 

 shallowness of the cup-like surface on the head of the radius. In fact, but for 

 this ligament the tendon of the biceps would be liable to pull the head of the 

 radius out of the joint. 1 In complete extension the head of the radius glides so 

 far back on the outer condyle that its edge is plainly felt at the back of the 

 articulation. Flexion and extension of the elbow-joint are limited by the 

 tension of the structures on the front and back of the joint, the limitation of 

 flexion being also aided by the soft structures of the arm and forearm coming in 

 contact. 



In combination with any position of flexion or extension the head of the radius 



FK;. 173. Sagittal section of the right elbow- 

 joint, taken somewhat obliquely and seen from the 

 radial aspect. (After Brau'ne.) 



1 Humphry, op. cit., p. 41 'A 



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