148 THE ARTICULATIONS. 



The articular surfaces which form this articulation are; in the ulna, the small 

 sigmoid cavity, a surface excavated in the lateral sense, and semicircular ; in the radius, 

 a cylindrical half-hinge received into the preceding cavity. 



To unite these there is an annular ligament, a kind of fibrous web thrown around 

 the superior extremity of the radius, fixed inwardly on the ulna near the inner 

 extremity of the small sigmoid cavity, attached outwardly to the external lateral 

 ligament of the elbow articulation, and confounded superiorly with the anterior 

 ligament of the same articulation. This fibrous web, in uniting with the fibro- 

 cartilaginous cap of the external humero-radial ligament, and joining the small sigmoid 

 cavity by its internal extremity, transforms this last into a complete ring, covered with 

 cartilage in its bony portion, and lined by synovial membrane that of the elbow 

 articulation in its ligamentous portion. The head or superior extremity of the radius 

 is also incrusted over its entire contour with a layer of cartilage: a disposition which, 

 permits it to glide not only in the concave face of the small sigmoid cavity, but also on 

 the internal face of the two ligaments which complete this cavity. 



Inferior radio-ulnar articulation. This is also a trochoid articulation analogous to 

 the preceding, but inversely disposed. Thus, the concave articular surface is hollowed 

 on the radius, outside the inferior extremity ; the convex surface lies within the ulna. 

 These two facets are very small, and are maintained in contact by a diminutive 

 peripheral fibrous capsule. A strong interosseous ligament, situated under the 

 articular facets, also consolidates this diarthrosis, and concurs by its inferior border to 

 form the antibracliial surface of the radio-carpal articulation. A small synovial 

 capsule is specially devoted to this articulation. 



Mechanism of the radio-t^nar joints. The play of these two articulations is 

 simultaneous, and tends to the &ame end; that is, to the execution of the double 

 rotatory movement which constitutes supination and pronation. 



Supination is when the ulna remains fixed, and the radius pivots on it in such a 

 manner as to carry its anterior face outwards. Its superior extremity then turns from 

 within forwards, and even from before outwards if the movement is exaggerated, in the 

 articular girdle formed by the small sigmoid cavity of the ulna and the ligaments which 

 complete it. The inferior extremity also rolls on the ulnar facet in describing a similar 

 movement, and the internal tuberosity of this extremity is carried forwards. 



In the movement of pronation, this tuberosity is brought inwards, and the anterior 

 face of the radius comes forward by an opposite mechanism. 



The inferior ray of the anterior member being articulated in a hinge-like manner 

 with the radius, it follows that bone in its rotatory movements, the anterior face of the 

 metacarpus looking outwards during supination and forwards in pronation. 



The radio-ulnar articulation in Man resembles that of the Dog and Cat, the articular 

 surfaces only being larger and the movements more extensive. In supination, the 

 palmar face is turned forward, and the radius, situated on the outer side of the ulna, is 

 in the same direction as the latter. In pronation, on the contrary, the palmar face of 

 the hand looks backwards, and the radius, remaining outwards in its upper part, crosses 

 the ulna in front in such a manner that its lower extremity is placed within the ulna. 



4. Articulations of the Carpus. 



(Preparation. Kemove the tendons from around the articulation, detaching their 

 sheaths, but taking care of the ligaments.) 



These comprise : 1, The articulations uniting the carpal bones of the 

 first row to each other ; 2, The analogous articulations of the second row ; 

 3, The radio-carpal articulation ; 4, The articulation of the two rows with 

 each other ; 5, The carpo-metacarpal articulation. 



ARTICULATIONS WHICH UNITE THE BONES OF THE FIRST Row TO EACH 

 OTHEE. These bones, four in number, are joined by the diarthrodial facets 

 on their lateral faces and form small arthrodial articulations. 1 They are 

 maintained in contact by six ligaments, three anterior, and three interosseous. 

 The anterior ligaments are small flattened bands carried from the fourth bone 

 to the first, from the first to the second, and from that to the third. The 

 first, placed outside rather than in front of the carpus, is covered by the 



1 The facet uniting the supercarpal to the first bone is not situated on one of its 

 faces, but rather on the anterior part of its circumference. 



