148 TBE ABTICULATIONS. 



' The articular mrfaces which form this articulation are: in tl^e uln% *^^^„ 

 sigmoid cavity, a surface excavated in the lateral sense and semicircular , m tiie 



'"''^^^S^^^tl^^lrt^^^l^^^i^ fibrous weh throwB ^ nd 



cS-tUa'lous cap o the external humero-radial ligament, and joining the small sigmoid 

 cavUy byZ int^einal extremity, transforms this last into a complete ring covered with 

 cartS-e in its bony portion, and Imed by synovial membrane-that of the elbow 

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

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

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

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



Inferior radio-ulnar articulation. —T!h\a 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 antibraoliial surface of tlie radio-carpal articulation, A small synovial 

 capsule is specially devoted to this articulation. 



Mechanism of the radio-ulnar joints. — The play of these two articulations is 

 simultaneous, and tends to the same 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 tliat 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. 



(Preparafe-o«.-Kemove the tendons from around the artiowlation, 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; 

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

 each other; 5, i he carpo-metacarpal articulation 



Aeticdlations which ukite the Bokes- of 'the First Eow to each 

 OTHEE-These bones, four m number, are joined by the diarthrodial facets 

 ontherr lateral faces and form small arthrodial articulations.^ TheTare 

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

 The anterior ligaments are small flattened bands carried from the fom'th bone 

 to the first, from the first to the second, and from that to the tWrd The 

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



' The facet uniting the supercarpal to the first bone is not situated on on« nf it, 

 faces, but rather on the anterior part of its circumference. °® °' ™ 



