326 



THE ARTICULATIONS OE JOINTS. 



flexed position the hand is carried medially in the direction of the mouth. Extreme flexion 

 is checked by the soft parts in front of the arm and of the forearm coming into contact, 

 and extreme extension by the restraining effect of the ligaments and muscles. In each case 

 the movement is checked before either the coronoid process or the olecranon come into contact 



with the humerus. 

 The anterior and pos- 

 terior bands of the 

 ulnar collateral liga- 

 ment are important 

 factors in these re- 

 sults. Lateral move- 

 ment of the ulna is 

 not a characteristic 

 movement, although 

 it may occur to a 

 slight extent, owing 

 to a want of complete 

 adaptation between 

 the trochlear surface 

 of the humerus andi 

 the semilunar notch 

 of the ulna. This 

 incongruence is note- 

 worthy since the 

 medial lip of the 

 trochlea is prominent 

 in front, and the 

 lateral lip is promi- 

 nent behind. Conse- 

 quently, this lattei 

 part is associated with 

 a surface on the 

 lateral side of the 

 olecranon which ie 

 only utilised in com- 

 plete extension. 



The capitulum and 

 the opposing surface 



upon the head of the radius are always in varying degrees of contact. The head of the radium 

 participates in the movements of flexion and extension, and is most closely and completely in 

 contact with the humerus during the position of semi-flexion and semi-pronation. In completed 

 extension a very considerable part of the capitulum is uncovered by the radius. 



Humerus 



Olecranon pad of fat 



Ulna 



Coronoid 

 pad of fat 



.Coronoid process 



Trochlea 



lecranon 



FIG. 307. VERTICAL SECTION THROUGH THE HUMERO-ULNAR PART OP THE 

 ELBOW-JOINT. 



THE RADIO-ULNAR JOINTS. 



These articulations, which are two in number, are situated at the proximal and] 

 distal ends of the radius and ulna. They provide an adaptation whereby the radius I 

 rotates around a longitudinal axis in the movements of pronation and supination. 

 and hence this form of uniaxial diarthrosis is termed lateral ginglymus. 



Articulatio Radioulnaris Proximalis. The proximal radio-ulnar joint forme 

 a part of the articulatio cubiti or elbow-joint. The articular surfaces which enter 

 into its formation are the radial notch of the ulna and the lateral aspect of the 

 head of the radius. In each case the articular cartilage is continuous with an 

 articular surface entering into the formation of the humero-radial and humero- 

 ulnar joints, consequently the joint cavity is continuous with the cavities of those 

 joints, and therefore, in a sense, it lies within the cover of the articular capsule 

 of the elbow-joint ; but its special feature is the annular ligament of the radius. 



Lig. Annulare Radii. The annular ligament of the radius (O.T. orbicular 

 ligament) (Figs. 305 and 308) has been mentioned above as the distal line oil 

 attachment of the radial collateral ligament and the ligaments on the front and| 

 back of the elbow-joint. 



It is a strong, well-defined structure, attached by its extremities to the volai 

 and dorsal margins of the radial notch of the ulna, and thus it forms nearly |s 

 four-fifths of an osseo- tendinous circle or ring. The circle is somewhat wider at I 

 the proximal than at the distal margin of the annular ligament of the radius; 

 which, by encircling the proximal part of the neck of the radius, tends to prevent 



