352 THE ARTICULATIONS. 



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 

 can be rotated in the upper radio-ulnar joint, carrying the hand with it. The 

 hand is articulated to the lower surface of the radius only, and the concave or 

 sigmoid surface on the lower end of the radius travels round the lower end of 

 the ulna. The latter bone is excluded from the wrist-joint (as will be seen in the 

 sequel) by the interarticular fibre-cartilage. Thus, rotation of the head of the 

 radius round 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 radio-humeral portion of the ioint 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. 



Surgical 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 disloca- 

 tion, 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 pro- 

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

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

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

 dislocation backward, while the olecranon loses its grip and is not so efficient, as during exten- 

 sion, in preventing a forward displacement. When lateral dislocation does take place, it is gen- 

 erally incomplete. 



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

 than dislocation of any other articulation, for, as a rule, fracture of a bone more frequently 

 takes place, under the application of any severe violence, in young persons than dislocation. In 

 lesions of this joint there is often very great difficulty in ascertaining the exact nature of the 

 injury. 



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 pro- 

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

 posterior ligament. Occasionally a well-marked, triangular projection may be seen on the outer 

 side of the olecranon, from bulging of the synovial membrane beneath the Anconeus muscle. 

 Again, there is often some swelling just above the head of the radius, in the line of the radio- 

 humeral joint. There is generally not much swelling at the front of the joint, though sometimes 

 deep-seated fulness beneath the Brachialis anticus may be noted. AVhen suppuration occurs the 

 abscess usually points at one or other border of the Triceps muscle ; occasionally the pus 

 diseharges itself in front, near the insertion of the Brachialis anticus muscle. Chronic synovitis, 



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



