ELB O W- JOINT 309 



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. When the forearm is extended 

 and supinated the axis of the arm is not in the same line as the axis of the forearm, but the axis 

 of the arm forms an angle with the axis of the forearm, 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 surface 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 inter- 

 nal condyle of the humerus and by the Brachioradialis; 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 constitute an enarthrosis, and 

 allow of the movement in all directions were it not for the orbicular ligament by \vhich the head 

 of the radius is bound down firmly to the lesser sigmoid cavity of the ulna, an arrangement which 

 prevents any lateral separation of the two bones. It is to the same ligament that the head of 

 the radius owes its security from dislocation, which would otherwise constantly occur as a con- 

 sequence of the shalloxvness 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 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 can be rotated 

 in the upper radioulnar joint, carrying the hand with it. The hand is directly articulated to the 

 lower surface of the radius only, and the concave or sigmoid surface on the lower end of the 

 radius travels around the lower end of the ulna. The latter bone is excluded from the wrist-joint 

 (as will be seen in the sequel) by the articular disk. Thus, rotation of the head of the radius 

 around an axis which passes through the centre of the radial head of the humerus imparts circular 

 movement to the hand through a very considerable arc. 



Surface Form. If the forearm be slightly flexed on the arm, a curved crease or fold with 

 its convexity downward may be seen running across the front of the elbow, extending from one 

 condyle to the other. The centre of this fold is some slight distance above the line of the joint. 

 The position of the radiohumeral portion of the joint can be at once ascertained by feeling for a 

 slight groove or depression between the head of the radius and the capitellum of the humerus at 

 the back of the articulation. 



Applied Anatomy. From the great breadth of the joint, and the manner in which the articular 

 surfaces are interlocked, and also on account of the strong lateral ligaments and the support 

 which the joint derives from the mass of muscles attached to each condyle of the humerus, 

 lateral displacement of the bones is very uncommon, whereas antero-posterior dislocation, on 

 account of the shortness of the antero-posterior diameter, the weakness of the anterior and 

 posterior ligaments, and the want of support of muscles, much more frequently takes place, 

 dislocation backward taking place when the forearm is in a position of extension, and forward 

 when in a position of flexion. For, in the former position, that of extension, the coronoid process 

 is not interlocked into the coronoid fossa, and loses its grip to a certain extent, whereas the 

 olecranon process is in the olecranon fossa, and entirely prevents displacement forward. On 

 the "other hand, during flexion, the coronoid process is in the coronoid fossa, and prevents dis- 

 location backward, while the olecranon loses its grip and is not so efficient, as during extension, 

 in preventing a forward displacement. When lateral dislocation does take place, it is generally 

 incomplete. 



Dislocation of the elbow-joint is of common occurrence in children, far more common than 

 dislocation of any other articulation. In lesions of this joint there is often very great difficulty in 

 ascertaining the exact nature of the injury. Sprain of the elbow is a very common injury in child- 

 hood. Injury to the radiohumeral joint is frequently produced by lifting a child by the hand, 

 as in swinging it over a gutter. The Supinator [brevis], which under normal circumstances would 

 retain the head of the radius against the capitellum of the humerus, is unable to do so, the radio- 

 humeral articulation receives the force and the orbicular ligament undergoes upward displace- 

 ment, is caught between the head of the radius and the capitellum, and jams the joint. This 

 injury is often called subluxation of the head of the radius. 



The elbow-joint is occasionally the seat of acute synovitis. The synovial membrane then 

 becomes distended with fluid, the bulging; showing itself principally around the olecranon process; 

 that is to say, on its inner and outer sides and above, in consequence of the laxness of the 



1 Humphry, op. cit., p. 419. 



